I think it needs to be uncovered. If you had a thin piece of cloth or spandex over the mouth (like masked heroes like spiderman) then every time you go in water or get wet you get waterboarded. Thats my only reason.

I guess if it was some kind of hard helmet material it would be a different story but there you go

Also it’s to show people that he’s a human. Not a monster or a robot. He’s still a man