Koby was captured by Alvida when he was 13 at most. Possibly 12. Smoker didn't get captured when Crocodile had a 2 billion dollar bounty. He got captured in his mid 30's by Crocodile with an 80 million bounty (albeit frozen tbf). Koby got captured at 18 by an Emperor with a bounty that's 40x greater than that.
Hate whoever you want. I'd argue with anyone that Koby is a better written character than Smoker. He's not as cool (At least he wasn't Pre-TS... post-skip Smoker's been put through the ringer), but the character's seen loads of development. There's much more to analyze with Koby than Smoker, which is kind of interesting since Smoker's been in more content and shown less. If Koby showed up and got his ass handed to him by Buggy, that wouldn't change the fact that he's well written. I'd say the handling of Koby is one of the first places I'd point to if I wanted to show Oda's skill as a writer, since he's not a character that we see a lot. He shows up every few hundred chapters.
In any case, characters will be as strong as they need to be. I've seen very few people place Koby on the level of a YC1, but if Smoker shows up as a YC1, he'll have made a massive offpanel jump without training or fighting anyone serious for a haki bloom. Law, Sanji and even Zoro all had massive booms without training or getting in serious fights. Luffy's one of the few characters who's had his growth explained. Characters will be as strong as they need to be.
From what I've seen, most people have Koby placed on the level of the Mid Trio which is fair. And the 500 million bounty has some people saying he's probably somewhere around Law or Luffy during those arcs (Which would still place him well beneath the stronger Supernovas, let alone Luffy).