I always felt this type of argument was a huge cop out against Groh.
His Costume is not anymore absurd than Maxi's, for example, it is just a garb with light armor on it, which for a assassin is nothing particularly absurd. Tira costume in III was actually her Bird of Passage Uniform tore down and Amy is dressed as a ballerina doll.
Personality wise, he has nothing to do with Z.W.E.I or Patroklos, the first was relaxed, playful and even solidary and the later was angry, self righteous and cocky.
From what we've seem, outside his interactions with Azwel, Groh is collected, cold and melancholic. You don't have to like him, but whenever people say it is because he feels out of place, it sounds like they want a reason to dislike him, rather than have a reason to dislike him.