Yesss XD
Your avatar is prooooo
I agree.
I'll give a detailed explanation sometime in the next few hours.
I play this matchup pretty much every day, and my older brother is a pretty decent Dedede.
Squirtle has a slight-to-moderate advantage. He can't get in safely on a grounded Dedede unless he powershields a Waddle Dee or a Forward-Tilt, but you can literally spam Water Gun on Dedede all day with a positive outcome.
If Dedede falls off the level, Squirtle will hurt him a lot. When Dedede gets the ledge, he's at a disadvantage, as Squirtle's F-Tilt beats out a normal multi-jump Dedede F-Air after the first one, and Squirtle can give chase after a ledge jump or ledge roll to knock him off the stage again. Dedede is also notoriously easy to footstool, so Squirtle can mix up annoying F-Airs and B-Airs with simple footstools. If Dedede is recovering, he's probably going to die, since Squirtle can pelt him with aerials until he's forced to Up-B, where he can restart the process all over again. Squirtle's D-Air has the ability to "kill" a recovering Dedede at as low as 90-ish%, as well; if Dedede runs out of jumps, you can D-Air him out of his Up-B, dealing significant-enough knockback to allow you to Water Gun the next Up-B.
The only thing Dedede has in this matchup is much better range on his grounded attacks and his grab. Dedede can kill Squirtle at low percentages, but he has no solid way to land those attacks if Squirtle just stays away from him. Dedede cannot chaingrab Squirtle, so getting the free 16% from a B-Throw is usually the best option from a grab. While Dedede has better range on his aerials, the fact that Squirtle maneuvers much better makes this point almost moot. It's usually a very long, since Squirtle can't kill well, Dedede is fat, and fatigue will set in fairly quickly. Still, Squirtle doesn't have to approach on the ground at all, which makes this a tough fight for Dedede. I'm going to say 60-40 in Squirtle's favor.
Ivysaur vs. Dedede isn't as bad for Ivysaur as one might think. A good Ivysaur will be doing everything in his power to prevent himself from getting grabbed, and he has good tools for that. Some lesser-known ways of doing that is short-hopped Razor Leaf and his incredibly-safe D-Tilt, which is sort of like a B-Air that doesn't move, but has good knockback.
Ivysaur will B-Air all day if it means that Dedede doesn't get inside...but it's a bit nerve-racking for both characters. If Dedede whiffs something on the ground, he eats a Bullet Seed, which is usually a free 30-40%. If Ivysaur whiffs something on the ground, he eats a chaingrab -> D-Tilt, which is usually a free 20-40%, as well as setting up for a potential edgeguard. As a result, it's usually a waiting game...and Dedede is better-equipped for it, honestly.
However, Ivysaur is well-equipped for killing a recovering Dedede. B-Airs essentially force another jump, and D-Tilt outspaces Dedede's F-Air/B-Air at the ledge while doing significant knockback. An Up-B from Dedede is just begging to get Up-Smashed by Ivysaur, as well, killing him at about 90%. Getting Up-B'd by Ivysaur is also a good alternative, and is a potent kill move.
And, while you don't need me to tell you, Dedede can edgeguard Ivysaur pretty easily with F-Airs and B-Airs. Simple and effective; if Dedede can use a throw to get Ivysaur off the level, he's in a good position to potentially take a stock.
I'd put this at about 60-40 or 55-45 in Dedede's favor, and that's mostly because of Dedede's speed (You don't hear that often, XD) over Ivysaur. This is mostly about who gets the other off the level first, and we all know how good Dedede is at that.
Charizard vs. Dedede is probably the most comfortable matchup for Dedede, because Charizard can't really space Dedede out via aerials. Charizard is going to be spamming Flamethrower all day, since Dedede can't really get around it. If Dedede is lucky, he might get a Gordo to go through the fire if he does it early enough, but otherwise it'll be shielded.
If he gets above Dedede, Charizard can't land on the stage safely at all, because none of his aerials are safe enough. Dedede should seek to get immediately under Charizard and grab him when he hits the ground. The only things Charizard can do is attempt to Flamethrower Dedede before he gets in range, or use Rock Smash as soon as he gets into range. Flamethrower is easily telegraphed this way, and Dedede should be able to roll behind and grab him with little problem. Rock Smash can usually be shield-grabbed, but if Dedede's shield is low, he might eat some damage instead. Since Dedede is a large character, it's not difficult for Charizard to space Rock Smash so that it deals that dreaded 45% from the shards. Either way, Dedede should be able to deal with a falling Charizard with little trouble.
Charizard can attempt to space you out with B-Airs, but any other aerial will be getting shield-grabbed. If Dedede throws a Waddle Dee while Charizard is on the ground, it's probably going to get powershielded, and Dedede is probably going to get grabbed.
Charizard's best bet for damage is going to be doing a F-Throw/B-Throw to get you off the level, and attempt to edgeguard. F-Air has massive gimping potential, so if a Dedede is recovering from high, you should try to land that for a KO. If Dedede is recovering from really high and you expect an airdodge to safety, a short-hopped N-Air from Charizard will always hit him when he lands. If Dedede is recovering from below, Flamethrower pretty much renders the edge ungrabbable, and if Dedede is caught in the fire, he's almost forced to take a large chunk of damage...and U-Smash is a reliable kill move around 120-130%. F-Smash is a given if Dedede lands on the ground from an Up-B. If you can outsmart Dedede, sometimes a D-Air spike is in order, as well.
Charizard is easy to edgeguard because of his large size and lack of helpful aerials while recovering. A couple F-Airs or B-Airs from Dedede will put him out of the game, or at least in a position where he has to land with Fly, which starts up another chaingrab. Charizard also has no reliable means of getting back on the stage from the ledge, as every realistic option can be shield-grabbed, so Dedede will take full advantage of that.
This is probably 60-40 in Dedede's favor. Charizard hates being in the air against him.
As such, the matchup itself is probably at about 55-45 in Dedede's favor. Hardly an easy fight, but it's doable on both sides if they play right. Mostly about who gets the momentum first.