Shouldn't the fat buu be called "Majin Buu" and the smaller buu be called "Kid Buu"
It's been a while, but here's what I recall...
There were actually two versions of the fat Buu... When he first appeared, he was Majin Buu. That is to say, he was Buu fused with a couple of beings from Kaioshin's realm, which made him appear fat at that time. (He was also pretty evil, in addition to being rather pure and childlike.)
Later, this fat and happy side was split away from his evil side, which created a purely good version that Satan named "Mister Buu," and also left his pure form "Buu" behind, unaffected by any fusions.
So I guess they call him "Mister Buu" in the game to say that he is separated from the original Buu... That would make sense, since they can actually fight each other. "Majin Buu" is actually the same thing as Buu, but with extra fusions inside of him. So having Majin Buu versus Buu wouldn't make sense since they're the same, but Buu versus Mister Buu does since they're totally separate.
(On a side note, buu is also the sound that a pig makes. Boo is cute, but it reminds me of "Boo! Did I scare you?")
