My guess is that he has several even to winning match-up against A and S-tiers.

vs. Sheik, he's pretty good. He mostly out-prioritizes her moves with his own. She's easy juggle food for him and a pretty simple to deal with recovery. A Bair off stage can kill her pretty easily. And he's small and light, so he avoids a lot of her follow-up and it can be torture trying to land a Fair or Bair on Kirby as Sheik. Every Sheik player needs to good Zelda against Kirby.

vs. Donkey, this is even in my opinion, but it's a wild match-up. Kirby can juggle the hell out of DK at any percent because of the low scaling on all of his Up moves. At 0%, Usmash > Usmash > Usmash > Uair string > Hammer for KO is a very easy BnB combo. The other side is that DK can kill Kirby at 50% with most of his moves, and kill at 0% with a Charged-Punch.

vs. Fox is probably like 70 or more for Kirby. Launch moves that give a ton of time for a hammer follow-up, a bunch of low-angled moves to gimp, and a good sort of high priority moves to edge-guard with (Bair, Dash-Attack, Fsmash). Fox has to be careful because it's very easy for Kirby to remove a stock in a single sequence.

He also fairs quite well against the other B-tier on this list, so there's that. In a lot of match-ups, if you're not a swordsman or lack disjoints, you can't hit Kirby during his Dash-Attack. The moves is incredibly stupid in priority and hit-boxes.

How is that related?

Lunary, have you tried getting a mechanical or ergonomic keyboard? My hands hurt sometimes when I program and use keyboard shortcuts or play Fox too much, and it's super helpful to get a mechanical keyboard. Just make sure that the keys aren't too hard to press so you can still short hop and stuff and you'll be much happier.