Honestly, most of the designs in Other M hit the mark pretty well. That being said, unmasked Nightmare looks godawful for a number of reasons, imo:
1. The pupils. They make the design look cartoony, which doesn't jive all that well with the design language of Metroid as a whole. A number of bosses in this game suffer from the same problem.
2. The other reason this design falls flat is because it fails to evoke the same visceral sense of disgust that the original had when Nightmare's face starts melting from within its carapace.

I personally feel that if those changes were made, this version of the boss could be a largely well received redesign.

Nah fam, fusion was way better than whatever kind of kids show this is

He actually looks kinda cool in this game, to be honest.