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.
(..) I heard it. I think that was meant for you. Even if Barton didn't tell Loki about the tower, it was still all over the news. The Stark Tower? That big, ugly building in New York? It's powered by an arc reactor, a self-sustaining energy source. That building will run itself for, what, a year? (..)2012 The Avengers