He‘s not, but the way the fight has been framed so far makes it pretty clear it’s Kid number one and Law number two.
Doesn’t make Kid massively stronger than Law or anything, the two are still neck and neck (and far better rivals to each other than they are to Luffy). But if I was to pick a main protagonist for this fight, I wouldn’t say “oh, the two are equal, I can’t pick between them”, I’d pick Kid.
So far, at least, we still haven’t seen all that much of the fight. The final hit might be a joint effort between the two for all I know. But right now, I’d expect more for it to be a doublespread of Kid punching her, and that’s usually the sign of the main fighter.
Plus I still don’t think they’ll beat Big Mom in a fight, anyway. Not unless more characters come to help. I think they’ll lead to her defeat and both be impressive, but I’m not expecting them to just plain knock her out in a good old punchup. Toss her off the island again, maybe.