ugly as sin

(as) ugly as sin — страшен как смертный грех

plug-ugly (very ugly)
.... a plug-ugly old bulldog
.... I may be plug-ugly, but I'm lovable.

Also
with a face like a bag of spanners
face like a burst couch
face like a dropped meat pie
face only a mother could love
face that would scare a dog out of a butcher shop
hatchet-faced (having a thin, hard, and unpleasant face)
have a face like the back end of a bus {UK informal} (to be very ugly)

so ugly, they'd have to sneak up on a glass of water to get a drink
ugly as the south end of a north bound horse
ugly as a hat full of holes
ugly as a mud fence

"hit every ugly branch falling out of the tree"
"she's been hit with an ugly stick"

see also
[ни рожи ни кожи]