I mean I'm not a character designer or anything, but I feel like the best monster girls have more monstrous traits than human traits. So Yiggdrasil, Mako, and Suzu are good examples cause they're humanoid, but with enough inhuman traits that you couldn't classify them as humans.
That's kinda why I think Fuyuko, Ara, and Typhon are bad monster girl designs, they just look like normal people with minimal monster traits. A lot more could have been done to make them look more like monsters, and less like normal people in cosplay.
Faranne, Shuri, Bob, and Ophelia are the only ones who can get away with having bad monster girl designs cause they aren't meant to be monsters. But then why put characters like that In a game that litterally has "monster girl" in the name? So I'd also avoid characters that aren't actually monsters (unless it's plot reasons or smth like that)