Oda is inconsistent as fuck, but I believe that, if Mihawk's bounty if after he joined the Cross Guild, that would make more sense of the "Yonko Buggy" and also would hype Crocodile more, IMO.
But the thing is....Law and Kid have 3B bounties, no yonko title, despite them defeating one, their crew suck big fat dick, aside from Killer and 0 territories as far as we know.
Mihawk getting a bounty above them doesn't even seen that absurd to me, honestly. There's also the fact that Mihawk is some endgame material. 1000+ chapters Oda REALLY needs to justify his hype.
I think a big fat bounty does the trick for him.