BMs head is just as great a prize as Kaidos. So he didnt settle for her.
But when the other SNs arrived on the rooftop kidd was talking shit saying they were there to watch him fight and he was the SN who did the least and now hes cheerleading Luffy.
Oh I am not arguing that BM being less valuable than Kaido as a prize or anything of that line, to me Big Mom = Kaido too.
But the problem to me is that Kidd went back on his word. He declared that he will be the one who takes down Kaido, yet he ended up taking a different path instead of walking his talk, which I was quite disappointed. Its a principle thing.
That too and also yeah Kidd shouldn't cheerlead Luffy, if anything instead of "I won't let big Mom reach the rooftop where Luffy is!" he should have at least be like "Fuck this, I will take down Big Mom faster than you Luffy, and I am coming for Kaido next!" That's what a rival's spirit should be like.