Major Edit: 2000 posts, Lolol
of Fox, Falcon, Mario, and Yoshi, Mario and fox are by far your best choices
Falcon is bad vs kirby but if you insist on maining falcon, spam short hop back air. Never approach a Kirby who is above you as down air spells death. Down tilt Kirby's up b when Kirby recovers. Kirby is Falcon's worst matchup overall fyi. Down air works well also if its not predicted. Up smash can out-prioritize Down air if you are not predictable. Pivot f-smash works well vs up tilt.
Yoshi is really bad vs Kirby. me and Sheer have both agreed it is Yoshi's worst matchup, don't even bother.
For Mario, do exactly what your opponent does, camp. But this time, camp with projectile. if your opponent is bad at smash DI, down air up air down air up air up smash is an easy way to kill. If your opponent has good DI, up air is still a good move and the up smash kills kirby around 80 I think. Force kirby to approach before you do. Recover smart, down smash Kirby's up b or back air for edgeguards.
For fox, spam projectiles, down air to up tilt if the opponent has bad DI. down air isn't that bad otherwise. experiment with using Forward air and n-air and fox's other moves into an f-smash.