Slight Disadvantage (40:60)
An argument could be made that the match-up is less in Kirby's favor straying into the "Moderate Disadvantage" (35:65 to 30:70), but I feel like the match-up either is not as bad as people make it or that the current meta has not quite reached that level of Diddy domination. I'm optimistic that this match-up might even prove to be close to 45:55 with enough R&D from the Kirby side of things.
However you look at it, even a moderate disadvantage against such a 'tournament threat' as Diddy is entirely winnable.
General Match-up
Diddy and Kirby are somewhat similar: they're both 'brawler' types that deal the bulk of their damage up close and personal. This is good news for Kirby since he traditionally has had trouble dealing with disjoints and projectile camping. Their air mobility is also relatively similar, and while Diddy may be faster on the ground it is not to such a degree as to make it especially difficult for Kirby to keep up with.
While Diddy is on the lighter side weight-wise, he is not 'floaty' which will give Kirby ample opportunity to take advantage of his combo game. Juggling Diddy with U-tilt is not difficult, and following up after juggles is feasible even after laying some damage on him. Diddy's light weight gives Kirby KO opportunity with his Smash Attacks or B-air without having to hope for bad DI or to have to deal heavy damage.
On the flip side, Diddy has a few attacks in his repertoire with deceptively good range such as F-air, D-tilt, and the infamous U-air. Diddy can also pull out most of his moves extremely quickly, which is something quite detrimental to a Kirby trying to beat him to the punch. This combination of speed and range give Diddy strong options against most of Kirby's offensive or defensive game.
Most of Diddy's strategy will revolve around his Grab game. By itself it's a good option since Diddy can force the opponent to Shield often, usually with his quick pokes or with the threat of a banana peel in hand. Not only does he have opportunities for a good Grab game, he has a
strong Grab game.
Diddy's D-throw is already well-known to Smashers around the world for it's quick execution and its easy conversion into various Aerial Attacks (specifically U-air). It's speed gives the opponent precious little time to react with proper DI and the follow-ups can either rack up damage (B-air to B-air at low percents) or outright KO (that U-air).
The Neutral Game
The main reason this match-up is not too bad for Kirby is because he can hold his own in the Neutral Game fairly well. Diddy has two 'ranged' options at his disposal during Neutral and those are his Peanut Pop-gun and his Banana Peel. Kirby is somewhat limited and will either try to close the distance or play the waiting game.
If Diddy goes for a banana peel this gives Kirby the opportunity to close the distance and try to get some stage control or pressure Diddy with threat of attack. If Diddy is far enough away to use Peanut Pop-gun (usually because he already has a banana peel in hand), this gives cause for Kirby to respond.
The general tactics to use against Peanut Pop-gun are: Shield, Jump, Duck, Item Grab or attack the peanut. It's usually a good idea to go for an 'Item Grab' and catch the peanut out of the air - this will eliminate the peanut threat and give Kirby a handy item at the same time. Staying back and being defensive can work if you want Diddy to be the one on the offense - Shielding or attacking the peanut works fine for this. In fact, Kirby's Inhale works great as an attack against incoming peanuts since they heal 1% each swallow (just mind the recovery time when eating peanuts since Diddy can take advantage of that). Jumping over one peanut works well, but chances are Diddy will fire another one right behind. In this case you can perform a Mid-air Item Catch as you approach.
Similar tactics can be employed against Diddy's banana peel. The difference here is that Diddy can use banana peels with greater offensive options: if you Shield he can run in and Grab (or feint a Grab), if you Jump he can strike with Aerial Attacks (or pressure you from beneath). Inhaling a banana peel is not generally a good idea because Diddy will usually be on the offense and swallowing gives him too big of an opportunity to punish. Catching banana peels is a great tactic. Doing so will eliminate Diddy's strongest ranged advantage, putting it literally in Kirby's hands.
While you carry Diddy's banana peel he will be unable to generate another one. Banana peels go right through Peanuts and will usually hit and trip Diddy while he's recovering from the shot. This means that while Kirby has a banana peel, he generally has a superior position in the Neutral Game.
Diddy's pressure in the Neutral is decent since he is able to mix it up with very quick attacks (U-tilt, D-tilt, throwing a peel), slower attacks with more range/power (F-tilt, F-smash, D-smash), or going for a Grab. Kirby's pressure is a little less threatening since his Tilts are shorter-ranged and his Smashes done have as much range/power, but the speed of Kirby's attacks can be a threatening presence to Diddy. Kirby also gets as much - and in many cases - more damage off his hits and combos than Diddy does.
One thing to watch out for is Diddy's Monkey Flip. This mix-up can 'grab' you from a good distance away, deals decent damage, and is hard to punish. Whenever Diddy wants to punish a Shield, he can launch himself at Kirby. This mix-up can be mixed-up even more by the fact that it can be changed into a kick. This means that even if you're predicting a Monkey Flip or even
reacting to the Monkey Flip, you have to guess if it's going to be a kick (Shield) or a grab (don't Shield). Often times Diddy will use a Monkey Flip when he has a banana peel in hand and expects you to put up a Shield in response - so keep an eye out for it!
Kirby at Disadvantage
Diddy will usually be putting Kirby into a disadvantageous position by keeping him under pressure of attack or threat of attack. When at a distance Diddy is trying to force Kirby to take damage with Peanuts or Banana Peels, or, at the least, force him into a more defensive position. This kind of pressure can create openings that Diddy can take advantage of, such as grabbing a Shielding opponent, striking a poorly placed Dodge, or intercepting an airborne opponent off-guard.
Diddy's pressure isn't limited to peanuts and banana peels, he has some good spacing tools. One of his prime spacing tools on the ground is his D-tilt. It's fast, hard to punish, and reaches a good distance in front of him. Moreover, if one hits it can lead into a strong advantage: at low percents additional D-tilts can chained together or a Grab mix-up can selected in case the opponent tries to Shield; at higher percents D-tilt can lead to a U-smash or U-air follow-up which will usually result in a KO on Kirby.
Aerial pressure is another option since Diddy has an excellent aerial game. His F-air can beat out many of Kirby attacks and can be Auto-Canceled on a Short Hop even with a Fast Fall option. B-air works the same as F-air but with less range and less damage output. The B-air, however, is much quicker than a F-air to the point that Diddy can throw a second B-air out of a Short Hop before touching the ground. This lets him string together multiple B-airs with a Hit-Confirm, gives him additional pressure with the threat of a possible second B-air, or allows him to Fast Fall and go back to his ground-based options.
One of Diddy's most threatening attack is his U-air. It's extremely quick, very safe, has excellent range, and can be combo'd from multiple set-ups. It's such a generally strong threat that it must be taken into account at practically any given point in the game. It can be used to build damage, to go for KOs, to force Air Dodges, to punish Air Dodges, etc. In fact, it's this singular attack that pushes the match-up in Diddy's favor. Without his U-air the match-up with Kirby would be even at best and most likely in Kirby's favor.
Off Stage Disadvantage
When Kirby is sent off-stage but not outright KO'd and Diddy is still on-stage, this is where Kirby has "Off-Stage Disadvantage". In general Diddy is not especially dangerous off-stage against Kirby, but it's still not a good position to be in and Diddy still has things to watch out for.
For the most part, Kirby will not have too difficult of a time getting back to the stage as long as he has multiple jumps available to him. Just watch to see what Diddy is doing - often Diddy will simply take the opportunity to spawn a banana or to shoot Peanuts as Kirby returns to the Stage. There's always the chance that Diddy will go aggressive off-stage in which case he will likely try to connect with a F-air or U-air though especially cheeky monkeys will try to go for the dunk with D-air as well.
Do your best to avoid any off-stage attacks and make it back to the stage. If Diddy whiffs any of his attacks off-stage he generally puts himself into a very bad position. At this point Kirby can stay off stage and try for some off-stage offense of his own, or he can simply get back to the stage where he will then have a stronger position.
Kirby at Advantage
The favorable position Kirby has with Diddy is within close proximity, preferably with Diddy facing away from Kirby. It doesn't matter too much if Kirby is facing Diddy or if he is facing away from Diddy since U-tilt hits behind Kirby and is often times the preferred attack to land. When up-close to Diddy, Kirby has most of his options available to him and choosing which one generally depends on what percent Diddy is.
At lower percents Kirby will usually want to land a U-tilt or Grab. At higher percents Kirby will usually have to settle for safer attacks that either lead to stronger advantage (U-tilt, F-tilt) or play it safe with pokes (Jab, D-tilt). Grab is a good go-to for mix-ups or for Out-of-Shield punishes.
Kirby's prime combo game is generally when Diddy is at 0% to 18%. Due to Diddy's vertical physics (weight, gravity, whatever), Kirby can usually combo multiple U-tilts together to build up damage. Finishing the juggle with an Aerial Attack works well, as does going for a 'hard read' and punishing Air Dodges or reckless attacks. Catching Diddy with a Grab at this percent range can lead to a "B-throw to B-air" combo. D-tilt is another possibility since it is a nice poke and can force a Trip for a good follow-up opportunity.
As the damage builds on Diddy combos mostly give way to follow-ups, mix-ups, or general positional advantage. For instance, launching Diddy with a strong U-tilt while he's at 50% might lead to a U-air follow-up, but it can also work quite well as general positional advantage allowing you to benefit from how Diddy chooses to respond to a potential follow-up. The higher the damage Diddy has on him the more beneficial it is to use safer attacks such as Jab, D-tilt, and F-tilt. U-tilt works as a safe attack, but it loses it's combo potential and should be weighed against other options (such as F-tilt or Grab) which may lead to a stronger positional advantage.
Don't play 'too safe' too often. Mix-ups are very important against Diddy since he's going to be using them against you as well - you don't want to be too predictable in your offense or defense. Prudent use of Aerial Attacks helps keep the opponent off-balance and keeps your approaches fresh. Aerial attacks can be as aggressive or offensive as the situation merits, such as risky D-air to catch a presumed Spot-Dodge or a retreating F-air to check the opponent's Shield. Aerial Attacks are also a better risk to take when Diddy is armed with a banana peel because even if Kirby is hit with the peel while airborne he cannot trip.
When Diddy hits 'KO potential' levels of damage (usually 100% and up), that's when Kirby can feasibly go for KO attacks. Of course you don't want to rush headlong at Diddy and start using Hammers and Forward Smashes. Such brute force is generally shut down by even novice players. Instead, be patient and punish with your strong attacks when the occasion presents itself. When erring at high percents, it's better to err with a safer attack than with a laggy one. If you want to play it safe and still go for KOs, your best bet is B-air (usually a retreating B-air) or to build the damage until you can KO with B-throw. Waiting too long to get the KO has it's own risks though since Diddy with Rage can be a very intimidating force to reckon with.
Off Stage Advantage
This is a very favorable position to be in for Kirby. Kirby's options are plentiful but you have to respond appropriately to your opponent's attempts at recovery. Generally opponents can recover high, go directly for the ledge, or go low and then try to recover up to the ledge vertically.
If Diddy recovers overhead there's isn't much Kirby can do as far as gimping him goes, but you can follow Diddy's attempt to get back onto the stage and build some damage on him. Catching him with Aerial Attacks works fine, or you can opt to catch him when he lands with whatever you can (generally a Dashing Grab works well as you follow him around).
Catching Diddy when he tries to go directly for the ledge is a little harder since Kirby's air speed is sub-par and Diddy gets a good boost when using Monkey Flip. D-air works great on Diddy, but the start-up makes it difficult to pull off. If you don't have much time to fight off-stage - or you would prefer not to take the risk - you can always go for a "Ledge Trump" and knock him off the edge as soon as he grabs it. Bumping Diddy off the ledge sets him up great for a B-air.
Recovering low for Diddy is a bad idea for Diddy, but a prime opportunity for Kirby. This is especially true when Diddy is recovering against a wall (as seen on many Omega stages). Kirby has plenty of choices for Diddy in this regard: N-air, F-air, B-air, and even Stone. Which one to use depends on the occassion. N-air works well to knock Diddy out of his Rocket Jet, tumbling to his doom. F-air and B-air are used for hitting Diddy against the Stage, or even for an outright KO out of bounds. Stone is a strong attack, but it has a vertical knockback which means you only want to use this if you think it will outright KO.