But Kirby as a counterpick? What MUs would Kirby be advantageous in that would warrant his use as a CP? Being a good CP=/=barely getting by in MUs, you should be winning those in theory
That's definitely true, so unless you're struggling with ZSS, Fox, and Wii Fit Trainer (lol), Kirby is probably better off as one of many pockets. Besides that, we're only talking about optionally played MUs. In the current meta game, the Kirby MU isn't that well known. So he has some tools he can abuse on top of surprises for the opponent.
I don't see how Kirby vs Ryu or ZSS could be even. If people say Sonic is godawful, I don't see how ZSS could be good with her ranged attacks, more aggressive offstage presence, and much more reliable kill set ups from aerials and grabs. As for Ryu, if fast, combo oriented characters like Fox, Pikachu, and Sonic can't claim an advantage against Ryu, I don't see what Kirby's got on them aside from his pink pancake crouch. Characters like Pikachu and Fox can run circles around him but they are also in kill range at like 50% (which is basically two combos and a well timed sneeze for Ryu) and Kirby's not exactly mobile. Again, I don't main him and I rarely see anyone use him so maybe some Kirby mains might want to open up the floor for this.
Quite simply, Sonic's strengths are entirely unique in the MU. The primary issue in Sonic vs Kirby is speed camping. An aggressive Sonic is an issue for Kirby, but not that bad because Kirby loves it when people approach him. Optimal Sonic play (see: Wrath) is the main issue.
ZSS's strengths aren't too painful for a grounded Kirby, because despite her mobility, she finds Kirby significantly harder to hit and keep a lead against. A missed grab can be punished by up to 50% damage, landing on Kirby is only asking for a usmash or utilt, and ZSS is light enough that she won't take too much damage before she's lost a stock. Offstage isn't great for Kirby, but multiple jumps allow him to avoid more offstage pressure than other characters. If he has to go low, it won't be great for him, though.
Ryu... I'm not sure about him. I do know that some Ryu's have been getting bodied by Kirby, but that seems to be MU experience more than anything. Emblem Lord says that Ryu needs to stay grounded in that MU, and it makes sense - plenty of Ryu's tools can be crouched and punished with uptilt. Or, if you're jumping at Kirby, just punished by uptilt. Ryu's grounded tools (especially dtilt) are important, then. It allows him to pressure and combo Kirby.
Either way, Kirby can hold his own there, despite dying crazy early. Not gonna give a number, because I have no clue.