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.
(..) Moaning Myrtle. -Who's Moaning Myrtle? -l'm Moaning Myrtle. l wouldn't expect you to know me. Who would ever talk about ugly, miserable. . . . . .moping Moaning Myrtle? She's a little sensitive. Gather round! Gather round. Can everybody see me? (..)2002 Harry Potter and the Chamber of Secrets