There is an old saying in Ukrainian folklore, which literally sounds like “[someone is] as beautiful as ass inside out” (“Гарна як срака навиворіт”). It is used when one wants to point a person's unattractiveness in ironic way, comparing it with something clearly not pleasant.
So I wonder what could be the equivalent proverb(s) in English with similar connotation, which can be used in same context. The proverb scheme seems to look like "as beautiful/attractive as [something unexpectedly nasty]". Any other not that precise, but idiomatic and figurative expressions with the same meaning would be appreciated as well.
To clarify the original word sraka (butt) and it's different meanings in Ukrainian (I translated it from Ukrainian, so there may be inaccuracies):
СРАКА, (sraka) 1. The bottom part of a human or an animal.
being unable to see smth as well as their own ass without a mirror (Не бачити, як власної сраки без дзеркала) - not to get something desirable
[Someone] was in the ass, saw shit (В сраці був, гімно видів) — about ignorant person
1.2. The bottom part which has unaesthetic look
Too look like a dead man's ass (Виглядати, як небіжчика срака); as beautiful as ass inside out (Гарна, як срака навиворіт) - to look unattractive
1.3. The bottom part of a human as indecent place that is embarrassing to display
1.4. The bottom part of a human to beat, execute punishment
Beat the ass for the head to become wise (Бий в сраку, аби голова розумна була) - about the appropriateness to apply physical methods of punishment
1.5. The bottom part of a human as the limited room, where one can go, be located, or to contain smth
[Someone] would get inside [another person's] ass, if he could (Він вліз би йому в сраку, якби міг) – about a person being sycophant, bootlicker
1.6. The bottom part of a human as the representation of their psychosocial characteristics
bare ass (з голою сракою) — with no money, no property
2. A person having negative qualities; // goof
drunk ass (п'яна срака) – drunk person
3. The bottom part of a human as the sexual object
4. Very complicated, hopeless and unpleasant situation
Source: Lesya Stavytska. Ukrainian Without Taboos: A Dictionary of Obscenities, Euphemisms and Sexual Slang