Summary
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Babidi never controlled Majin Buu because Buu's chaotic nature can make him immune to mind control.
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Fat Buu's innocence and purity, a result of absorbing a large supreme Kai, may have made him immune to mind control.
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Although Babidi's magic seal didn't use mind control on BUU, it was a decent deterrent that kept him loyal for a while.
The evil sorcerer Babidi z Dragon Ball Z was a unique villain in that he did not use brute strength, speed, or a unique fighting style to defeat his opponents, opting instead to use deception and magic. One of his most powerful spells is the ability to turn anyone with evil in their heart into his slave. He had done this with the previous Demon King, Dabura, and even Vegeta, who had started to turn over a new leaf, was vulnerable to his control. However, for some reason, Babidi did not use his mind control power on the most obvious target: Majin Buu, whose disobedience led to his death.
Majin Buu is practically the embodiment of evil in the Dragon Ball Z;; So you would think he has absolute evil in his heart to control Babidi. For those wondering if Babidi has already tried this, only to have Majin Buu break free from his control, he most likely didn't try it at all. IN Dragon Ball Z Episode 229, “Vegeta's Pride”, when Babidi saw that Majin Vegeta had refused his order to kill the Supreme Kai, he said that he had never experienced anyone rebelling under his control. Babidi's claim means that BUU has failed to control it before. But is it because he just didn't think to try, or because he already knew it wouldn't work?
“But- but that's never happened to me before! Since when can a possessed say no to majet?” – Babidi when Majin Vegeta refused his orders

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Possible reasons why Babidi never controlled Majin Buu's mind
Majin Buu might just have been too messy
While the manga and anime never directly addressed why Babidi and Bibidi couldn't or never tried to use mind control on BUU. However, you can pick up a few clues from the rest of the rest Dragon Ball franchise to create some theories.
One theory is that Majin Buu is a being of pure chaos, so maybe a mind control spell couldn't affect someone like him. Even Marba, the creator of Majin Buu, said Dragon Ball Daima that she couldn't control him. He even considers Majin Buu a failure of creation due to his chaotic nature.
Fat buu in particular may not actually be evil
Several markers point specifically to Buu's fat, which is a poor target for Babidi's mind control. Fat Buu is the result of Kid Buu absorbing the Grand Supreme Kai. The purity of the deity turned Buu into an innocent, carefree being. He's still incredibly powerful, but essentially no longer evil. Some might argue that absorbing the great supreme Kai removed Buu's evil completely and therefore Babidi's spell wouldn't work on him. When Mister Satan confronted Buu and asked him why he kept killing people, he asked him if it was a bad thing. Fat Buu didn't really know what he was doing was evil and was simply following Babidi's orders.
Admittedly, this theory is flawed. That means baby Buu, who is Pure evil, it could have been controlled by Bibidi, but clearly wasn't.
Perhaps Babidi thought the magic seal was enough of a deterrent
Although he didn't use his mind control powers on Buu, it's not like Babidi doesn't have a way to tame him. Babidi's greatest tool against the BUU was the magic seal developed by Bibidi. Whenever Buu got too rowdy or started disobeying orders, Babidi would seal him in a magic ball and sometimes the threat of being sealed would make Buu think twice.

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Dragon Ball has a history of ignoring Hax or “Instant Win” Powers
As long as there is an opponent they are using on
The biggest possible reason why Babidi can't use mind control on Majin Buu is because BUU is just too powerful. Dragon BallThe power-up system is unique in that even when a character has a power that is conceptually incredibly broken, the overall power still matters more. If a character with an arrested ability is facing an opponent whose power level is far above them, the power simply won't work against them. If Vegeta could break free and disobey Babidi, then Majin Buu, being stronger, would almost certainly do it, and with less effort too.
Another big example of this phenomenon was during the battle between Hit and Jiren during the Tournament of Power. IN Dragon Ball Super Episode 111, “The Extra-Dimensional Final Battle! hit vs. Jiren!”, Hit revealed his secret weapon, an evolved form of his time skip: Time Prison. This new ability completely restricts the target's movement by suspending them in time. However, Jiren freed himself from this by simply being “too strong”. Yes, Jiren managed to be immune to being suspended in time using brute force. It makes almost no sense logically, but so do the laws Dragon Ball work.
This technically happened with Super Buu vs Vegito as well. During their battle, Super Buu turned Vegito into a piece of chocolate with his candy beam, and to be fair, it he did work. However, even as a piece of candy, Vegito still had full agency over his being and could still move and fight. Super Buu had so much trouble fighting the little chocolate ball that he turned vegito back to normal.

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Dragon Ball Z is available to stream on Crunchyroll.

Dragon Ball Z
- Release date
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1989 – 1995
- Directors
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Daisuke Nishio
- Writers
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Akira Toriyama, Takao Koyama