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(be) a sight to behold AND (be) a sight to see (spectacularly beautiful || incredibly ugly or revolting, etc.) — глаз радуется; любо-дорого посмотреть (одобр., ирон.); загляденье; зрелище, на которое стоит посмотреть; ≅ отворотясь не налюбуешься

(as) ugly as sin -- страшен как смертный грех

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
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]