He may have mana equal to gods, but he's still human... Gods and Goddesses are divine beings, basically, they don't have any physical forms (unless they make one of course) which is why they cant turn into anything... MC's purely human with mana like gods, basically, he's a demigod.
(besides, he's good the way he is... xD and that's me pep talking)
I just think at first when he decided to wear a mask, he was unable to speak the language and looked different... which would make him alienated and marked as a demi.. but now alot of people know him so there is no need to keep wearing the mask..
And I think somewhere in the novel he stated that he is proud "kinda" of his looks as he was just average in Japan and that it's just the world of the goddess has strange standards and not to forget only humans regarded him as hideous ヽ(`Д´)ノ I respect him for that
I mean....if he has as much mana as the goddess, he can create an entire realm out of nothing, and he can basically do any spell as long as he imagines it. Shouldn't he be able to change his appearance at will? In fact, a simple remedy would be an ulra powerful illusion spell, where he has it constantly on, and anyone who looks at him perceives him as a better looking guy. He has the mana of a god, I'm pretty sure a spell like that could be sustained no matter what.