Majin Buu was once one of Dragon Ball Z’s strongest characters, so why isn't he shown that way in Dragon Ball Super? There was a time in the anime when he was regarded as the Z-Warriors’ greatest threat. Vegeta, even with an evil upgrade from Babidi, couldn’t kill him, and Goku himself only had a slight edge over him after going Super Saiyan 3.
By absorbing others, the evil Buu was able to beat powerful characters like Gotenks and Ultimate Gohan. Following his transformation into Kid Buu and his death at the hands of the Spirit Bomb, the good side of Buu remained in the anime. Though not quite as powerful as the wicked Super Buu, the character continued to be seen as a force to be reckoned with. He was counted as ally – albeit an unreliable one – in Dragon Ball Super. He briefly tussled with Beerus in the Battle of Gods Saga, defeated Universe 9’s Basil in the Zeno Expo, and was added to Goku’s team for the Tournament of Power. Unfortunately though, Buu’s unforeseen nap caused him to lose his roster spot.
Buu may have missed out on Dragon Ball Super’s largest battle, but enough was seen of him in the show to provide a good indication of his current power level in the anime. Prior to the tournament, it was confirmed by Goku that Buu (who received a slim new look) has grown stronger since Dragon Ball Z ended. However, his sparring match shows that the best he can do at this point is fight on par with Goku in his base form. So while he has increased in power, the gap between him and the top two Z-Warriors has drastically widened. With their new Super Saiyan God and Super Saiyan Blue transformations, Goku and Vegeta have left him far behind. Ultimate Gohan has easily done the same, as he now has the capability to push Goku into using his Super Saiyan Blue Kaio-ken power-up.
Buu doesn’t have enough strength to compete with Goku, Vegeta, or Gohan anymore, but at least through most of Dragon Ball Super, he was viewed by the Z-Warriors as one of their strongest. He was among the first characters Goku turned to when looking for allies in their contests against Universe 6 and Universe 9, as well as the Tournament of Power. That’s because Buu is still above several of the heroes, including Android 18, Krillin, Tien, Yamcha, Master Roshi, and of course Chiaotzu.
That being said, Buu’s loss of relevancy and failure to make significant gains have allowed some characters who were previously in his shadow to surpass him. Future Trunks and former villains Frieza and Android 17 are now on the level of Super Saiyan Blue. Piccolo, based on his ability to keep up with Ultimate Gohan in the Tournament of Power, also seems to have eclipsed Buu, despite the fact that he wasn’t even considered a threat to him in Dragon Ball Z. As for what the future of the Dragon Ball franchise holds for Buu, it’s possible that with a little training, he can stop any more Z-Warriors from outdoing him.