Ahh kirby, the pink fluffball of doom.
THe main thing Kirby players will do at low %s is try to grab combo you. THis combo is F-throw, uair, reverse U-tilt...and idk what passed that. However, because of Pikachu's light-weight nature, it is possible to get out after the uair. If you DI with both sticks up right when they hit you, you can jump out of it
. This will help TREMENDOUSLY seeing as comboing is one of the things that kirby players use to destroy other characters. FOr pikachu, you can still do basic FF Fair u-tilt to uair to whatever. So, while pikachu can get out of kirby's main comboes (Remember, you will still get hit by D-throw to utilt, and F-throw to uair, but that is it) pikachu can still combo decently well.
In this matchup, you should stay away as long as possible. Kirby has no projectile at the start of a match, and you have longer range attacks. Keep kirby at bay with your F-tilt, D-tilt, and T-jolts. The minute he gets into you, the match completely changes. Hell, if kirby gets close, try to QAC away and keep spamming. Just watch out kirby doesn't get your power, because Kirby T-jolts are more effective than pikachu's. With kirby's multiple jumps, it can be VERY irritating to get near him, and even then it is a hard fight.
Once the fight is in close quarters, your best options are to SHFF fairs to u-tilts and combo him. Do not do many laggy attacks against kirby because he will definitely punish you with his plethora of strong attacks, or get you off the cliff (will elaborate on this a little later) Kirby's like using D-tilt to trip you (It's pretty fast so be watchful of it) and then they can grab you and harass you.
In the air, this matchup all depends on positioning. If you are behind kirby, you will lose, becasue kirby's bair outprioritizes nearly everything you have! If you are in front of kirby, you will most likely win since Pikachu's fair > Kirby's fair, and pikachu's Uair > Kirby's fair. If you are below kirby, be watchful of his Dair. It is a multihit attack and keeps you in stun-time for quite a while, and i don't remember whether or not the kirby can F-smash you afterwards. Kirby's will almost never use their nair, for it is pretty bad. It had decent range, but ****ty knockback, and they have much better aerials at their disposal. Also, if you are below kirby, always be watchful of his Down-B, the rock attack. It packs a punch and will kill you at around 100-110%.
Off the level, kirby has the clear advantage. He can keep you off the level with his Bair, and it can be really troublesome to get around it. Also, if he sees that you are below him, he can FF a dair to footstool, getting him a near guaranteed kill at any %s, so be afraid, be VERY afraid. Thunder guarding works pretty well against Kirby, so use it when they are off the level, but don't get too predictable with it or they can just wait it out with his multiple jumps. Don't try to hit him out of his up-b, for it has lots of priority and it is extremely hard to do so. Hopefully you will get lucky and hit him with the blue part of thunder when they are off the level, killing them at 60%, but don't get used to killing them like that, its very rare.
In killing, Kirby clearly has the advantage. Although both are lightweight, Kirby's feet stole ganondorfs power, and make it insanely strong. F-smash will kill you at 105% or less, D-smash at about 120%, and U-smash at about 120%. Kirby also has his hammer, which when used in the air, swings twice with devastating results. Be very careful when you are at high %s pikachu's, you will need to get all the damage you can get on them. Also, bair will kill you off the level at about 90% fresh, and rock will kill you at no more than 105%. The most reliable way to get a kill on Kirby is FF fair U-tilt thunder, which will kill him at about 80% from the beginning of that little string. F-smash is very hard to hit kirby with, but if it does, it will kill at about the same time kirby's will (if it's sweetspotted). Nair will kill off the level at about 110%, and blue part of thunder will kill at 70%. However, Kirby's killing moves are much easier to land on pikachu, which is the main thing that gives him the edge.
I say this matchup is dead even or 55-45 in pikachu's favor, only because he can camp. Kirby has his advantages, and pikachu has his, and it makes for a very interesting matchup