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.
(..) Shoot. Shoot. Shoot. Joker... ...we're gonna have to put you up for the Congressional Medal of Ugly. Hard-core, man. Fucking hard-core. We have nailed our names in the pages of history enough for today. We hump down to the Perfume River (..)1987 Full Metal Jacket
(..) I'll be watching you. Did your parents have any children that lived?. -Sir, yes, sir. -I'll bet they regret that. You're so ugly you could be a modern-art masterpiece. -What's your name, fat-body?. -Sir, Leonard Lawrence, sir. Lawrence?. Lawrence what, of Arabia?. -Sir, no, sir. (..)1987 Full Metal Jacket