Buu's propobly the most underpowered character in the game. His only real bonus is his high HP. I mean... Look:
Mouthblast: Ok, this is a good wave, if not as good as a Kameyameya wave.
Candy: Comparable to a laser, but extremely weak. If your lucky enough to swallow someone you only than get a bonus.
Heal: In the show Buu regenerated in a mere second, here it takes him a mere lifetime to regenarate. Also when you do regenrate, people blast you with waves and what not.
So his attacks clearly suck. Never the less, if you know combos (I know one), hese excelent in mellee, untill the monkies turn Gold and your left dealing 1 damage to everyone! Than you resort to mouthblast and... Well he has a clear disadvantage. Since SuperBuu is coming out in the next version (and maybe KidBuu), Im sure his HP will be well balanced. But he does need a few more attacks, atleast the gut attack so that way you can actually use Candy. The only way you can use it now is if you toss someone into the ground and find yourself lucky enough that their HP is extremely low.
Buu needs atleast a good blast or laser, even though I havent seen him use one. Giving him a Kameyameya wave would pretty much give him another mouthblast.