It's interesting because yeah, going by the dialogue structure alone, it's not left any sign that South Kaioshin changed something in Boo's personality... The focus was always on Dai Kaioshin.
Still tho, while Buff Boo could logically be “Kid Boo + even more power + the same unpredictable behavior,” still, the plot emphasizes that the true, baseline evil is kid Boo... Seems more like a storytelling choice the way it's all structured — kid Boo is the primordial menace, while Buff Boo is, at the end of the day, more like a transitional step on the way to Fat Boo...
Kibitoshin : “First, two of them were killed… the Northern Kaioshin and the Western one… And then the one who was the most powerful, the Southern Kaioshin… unthinkably, he was absorbed by Boo..."
Old Kaioshin : “…So he turned into that big Majin Boo we saw just now?”
Kibitoshin : “Yes…”
Kibitoshin : “Next, the one who was absorbed was the chubby but gentle, mild-mannered Dai Kaioshin… The Boo that Bibidi created was pure evil, a failed creation that not even Bibidi himself could control. But by absorbing Dai Kaioshin, Boo at least became manageable…”
Dai Kaioshin is emphasized as he was the one who made Boo controllable. If you think about Bootenks, Boohan, they wouldn't be controllable either, they just, wouldn't be chaotic unpredictable like kid Boo... Still, it doesn't mean Buff Boo was every bit as chaotic, just not enough to be tamed by Bibidi, like how Super Boo or Bootenks, Boohan wouldn't, either. "The Boo that Bibidi created was pure evil, a failed creation that not even Bibidi himself could control. But by absorbing Dai Kaioshin, Boo at least became manageable…” that's referring to kid Boo, the one who was created. He is the one who is lacking any trace of rationality, the problem child. Buff Boo isn't said to have became more controllable, but again, it's because he was still a danger regardless, he was still evil just like Boohan was, which doesn't = lacking any reasoning in terms of just doing whatever in "screw it" way.