an 'ugly duckling' that has become a beautiful swan [=an ugly/unsuccessful person or thing that has become beautiful/successful]
('informal') That dog of yours is '(as) ugly as sin'. [=very ugly]
The house is an 'ugly duckling', but it has a lot of potential.
(..) We'll always be freaks, and we'll never be like other people! And you'll never be a freak 'cause you're just... too perfect! Yeah, well, at least I'm not ugly. Yes, you are. And you're boring, and you're totally ordinary, You two deserve each other. Jesus, man, (..)1999 American Beauty