I think it depends on to what extent someone is into cartoons, and what the attraction is. I used to watch MLP, as it made for interesting analysis into what morals they were trying to teach kids and how they went about doing it. It gave me ideas for how I might write children's literature, and the characters and the stories were cute and charming. And I wouldn't say it was any more childish than say, Harlequin romance novels, which plenty of "adults" read. Or wrestling. I don't think it's childish to like seeing simple moral lessons told in creative and visually appealing ways - it's the same appeal as super hero movies or morality plays or Aesop's fables.

But "Brony" spaces still disturbed me, a lot of it seemed more about objectifying the characters(because of course, they were still men) or of reading some really gross shit into something intended for children, which is inherently creepy. Developing an obsession with anything is very different from appreciating it as a work of art or storytelling, and that's the primary problem I had with Bronies or the anime-queer crowd. If it's the ONLY thing you watch and you can't tell that it isn't the same as real life - yeah, that's an issue.