Well I think that's the best reason I can think. Humans would be more likely to accept somebody who looks like them, even if it is a robot. If Mettaton-kun had appeared towards the masses like, I don't know, a eleven-foot tall green goblin I don't think fame and love would be what he would receive.

...maybe Mettaton isn't even his real name.

[Mouri is a bit annoyed Sigrun has the answers and he doesn't. It is understandable she would, at least]