When Rocky is in the funhouse looking at himself, he appears to look relatively "normal". But it's very obvious that it's got nothing to do with the mirror, and more to do with the removal of a lot of makeup. For example, Rocky usually has almost no nose, but in the mirror he has Eric Stoltz's very well developed nose.
Director Peter Bogdanovich's first choice for the part of Rocky Dennis was Anthony Michael Hall. Hall turned down the part because he didn't like the idea of hiding his face during the whole film with make-up. Eric Stoltz was then cast instead.