Yknow, Kirby comes up in these power scaling conversations a lot, but I've never seen Batman vs Kirby before.
My gut instinct is that Kirby could defeat Batman, but it wouldn't happen, because Batman would quickly figure out that he can get Kirby to do anything by dangling a lollipop in front of him.
I'd love if a Batman fan had a more nuanced take, though. I only know The Animated Series and it's not exactly fresh in my mind.