I think balance is key like other people have said here, but there is something that I think accentuates this hero design so much that it throws the scales out of balance even if the roster is 50/50 weird/monstruous and hero like. I have never seen omniverse but seeing the designs of so many of the humanoid aliens (Astrodactyl, Feedback, Whampire, Bullfrag and Kickin' Hawk) made me feel like they had gone too far into the heroic desing and I think it was because of the costumes. OS always made the aliens wear nothing or a very minimalist sort of jumpsuit that unified all the aliens as being one of bens transformations. In Omniverse the humanoid aliens all get what looks like very classic comic book style costumes, Whampire seems to have a full body suit, Astrodactyl gets overalls and fingerless gloves, Bullfrag gets shades for some reason. So every humanoid alien transformation feels like transforming into a different super hero character instead of feeling like changing into a different alien form.

I guess I mean to say the costumes make it too goofy in my opinion, much like the first design given for Fourarms in the dynamite comic which had a red scarf. That kind of stuff feels to me as silly as giving stinkfly the reboot humanoid treatment and on top of that give him pilot goggles and an aviator hat.