As others explained, Buu's relationship with Mr Satan appears to be the only anchor fastening him to the Z group. It's not as though he's struck up any conversations with anyone else or tried to kindle a bond except when it comes to Bee. Not even Videl appears to hold much regard for his well-being and it would seem like the two of them should be interacting more since they both happen to be closely associated with Mr Satan. Goku only allowed Mr Buu to live out of courtesy for helping them during the battle against Kid Buu. His continued existence means nothing to anyone except the false idol.
You could say preceding characters like Piccolo and Krillin have hit their stride. I wouldn't deny this. The staff seem reluctant to perpetuate their relevancy beyond some fleeting ancillary moments in the RoF and recent arcs against generic henchmen. We're tossed bones on occasion to sate our desire for sensationalist action from classic favourites. It never amounts to much else than a generous bonus, though. Buu really is no different and in fact you could argue that's he in some ways worse because the show falsely advertises an increased role for him only for a bait & switch to occur with some other character seizing his screen time for themselves. At least Piccolo and Krillin get to partake in some choreographical action against the weaker enemies. Buu's screen time has been scarcely thin.