This matchup is very very interesting...
Kirby has the known f-throw -> uair combo, but that is ALL he can get with correct DI. Smash DI the Uair up and you get out of the combo with a jump. Pikachu can also combo kirby very little, probably just with F-throw Usmash at low %s, and FF fair to utilt, and possibly to thunder after (i would save that for killing though)
In the kill department, kirby has it way easier. If the pika is at 80%, they will die from a sweetspot F-smash, 90% from u-smash, and 100 from D-smash. Pika must camp as hard as he can when at these %s to try and rack up damage before they die. Pikachu's sweetspot F-smash can kill kirby at around 85% from the center of the stage, and, sadly, thats the most practical way of killing kirby. Unless you get really lucky with a nair off the level when kirby is at like 100%, you need that F-smash or a godly luck thunder. However, setting up that thunder is near impossible since kirby's range > Pikachu's u-tilt range.
Spacing is THE MOST IMPORTANT THING IN THIS MATCHUP. Pikachu's D-tilt range > all of kirby's attacks except maybe hammer...idk. Pikachu needs to play very defensively in order to win with good spaced Dtilts, Ftilts, and maybe some SH dair past him. If the pikachu gets too pressured, they NEED to QAC away and T-jolt, breathing room is really necessary for pikachu.
What the matchup will most likely come down to is positioning. In this matchup, Kirby's ground game < Pika's ground game < pikas air game< kirby's air game. For the most part, the kirby will be in the air trying to knock you off the level or trying to put you into positions for F-smash. Both characters will probably be trading hits, which is why it is so important to get a few T-jolt hits to keep the advantage in pikachu's favor. Hope to god that kirby doesn't suck you up and get your power. If he does...Pikachu must go on the 100% total offensive. Kirby is MUCH better at camping than pikachu because of his multi jump.
Off the level is an interesting battle ground. It all depends on the positioning of the characters. If pika is behind kirby, kirby wins. If pika is in front of kirby, pika wins. If kirby is above pikachu, kirby wins. If pika is above kirby, pika wins. Both characters have fast and deadly aerials, so it all comes down to which are able to be executed from their positioning. Dair to footstool still works on pikachu, so if you are below kirby, try your ABSOLUTE HARDEST to get back to the level at an angle that kirby won't hit you with that.
Overall: 50-50, With even players this matchup is quite interseting. A mismatch on either sides leads to a rapefest. It's basically about who knows the matchup better and who is willing to be more patient.