To beat Kaido's equal vs Zoro degraded from rooftop to a Katakuri level opponent.
Big Mom is not Kaido's equal at all, lol.
Realistically, King was the only logical narrative option for Zoro as long as Oda wanted to stick to Straw Hats getting 1v1's and Luffy getting to 1v1 the main villain.
More importantly, Zoro ended his battle having surpassed King entirely. There is almost no way to argue that Law or Kid individually surpassed Big Mom in this fight whatsoever.
Without ACoC/King of Hell mode, Zoro already proved that he can damage Yonkos. It's not really a stretch at all to think that he would have performed similarly well if he replaced Kid or Law in this fight. In fact, I would argue that if you replaced Kid with Zoro they would have ended the fight much sooner.
Regardless, this fight was impressive for the 2 of them. Despite Big Mom being incompetent, Kid and Law still showed enough to elevate their status in my eyes. I still place Law unequivocally above Kid, but that could change depending on what the aftermath of his final attack looks like.