Since Dragon Ball FighterZ takes place during the Dragon Ball Super timeline, Kid Buu isn't really in the story. Since his soul was reincarnated as Uub, only a clone could be created.
Just a mindless minion of Android 21. He's kind of like a sub-boss in every level, so get used to seeing and fighting him a lot.
In multiplayer matches, you can play as him. And I mean the real deal, not some clone.
Though I wasn't too good with him. Though I met a few good human opponents who did really well with him. In the story mode, he wasn't too challenging. I'd always take him very seriously, so I'd never let him get the upper hand.