Usually refers to men. Especially men who don't pay much attention to their looks, in terms of grooming, hair and clothes.
However when they put some attention to it they look good.
Best example is
Shaggy from Scooby-Doo. He always looks frumpy, stained, unshaven, slump, but you always
get the feeling that when he needs to escort Daphne
Blake to her gf wedding
Shaggy after a shave and a nice getup will look good.