The first reason would be how unoriginal of a threat he is. In premise, focusing on a magic user is a good idea and would make up for the Boo Arc's untapped potential. Moro, however, doesn't perform this as his abilities essentially just make him have a more haxed version of Gero's abilities and the whole eating planets thing is a pretty blatant Galactus knockoff.
Secondly, his very existence is a plot hole that damages the franchise's credibility even more. The fact he wasn't released in Trunks' timeline makes no sense when none of the differences would result in Cranberry being prevented from freeing Moro. This is one of the major flaws in Super constantly introducing new threats from Universe 7 in a series that has branched timelines.
Perhaps the main reason though is how bland his personality and goals are. His merciless, opportunistic nature is no different from Piccolo Daimao, Saiyan/Namek Arc Vegeta or Freeza, all of whom have far more going for them. Speaking of Daimao, one can't ignore the identical goal they had in restoring their youth. The difference is Daimao was so powerful in his old state that the threat of him regaining it was easily conveyed, he had unique traits to his villainy such as the anger of feeling his sons die and he had many plans in place for after he regained his youth and conquered the world. Moro, meanwhile has none of that, the only reason he didn't get killed immediately being Goku and Vegeta's intelligence getting plot nerfed and he seems to have no goals beyond eating everything. What's more, he had the potential to be decent if him eating planets was more like Galactus in that it was something he had to do in order to live rather than just doing so for fun. Unfortunately, him being locked up for 10 million years snuffed that idea out from the start. It'd be understandable if Toriyama was writing this as he said he doesn't like to get psychological with his villains, but this was Toyotaro's first true chance to show his ability to craft a story and all he showed was how much of a hack he is.