Green’s Dictionary of Slang

ugly as... adj.

also uglier than…, (so) ugly that...

(US) used in combs. meaning very ugly or unwell-looking.

[US]J.K. Paulding John Bull in America 302: The women all as ugly as sin.
[US]D. Crockett Narrative of the Life of D.C. (1934) 39: She was as ugly as a stone fence.
[US]T. Haliburton Season Ticket 194: The other [daughter] is as ugly as sin.
[US]Interior Jrnl (Stanford, KY) 4 Aug. 1/5: She may be as ugly as a mud fence jeweled with hop-toads, clumsy as a cork-leg or a club-foot, and stupid as a boiled owl.
[UK]G.F. Northall Folk-Phrases of Four Counties 11: As ugly as sin. Said of an ill-favoured individual.
Herald (Los Angeles) 28 Oct. 9/1: He was ugly as sin .
[UK]Shields Dly Gaz. 8 June 2/6: Bill [...] was as mild as a kitten when he was sober, but ugly as a mad bull when he had a few drinks in him.
[Aus]Bulletin (Sydney) 26 Nov. 35/1: ‘He’s ugly as a jew lizard,’ answered Susy with vigor. / ‘Snakes alive! Ugly, is it!’.
[US]J.W. Carr ‘Words from Northwest Arkansas’ in DN III:i 99: ugly as a mud-fence, ugly as a mud-fence stuck with tadpoles, (so) ugly that he’d stop a bread wagon, (so) ugly that he’d stop a nigger funeral, adj. phr. Extremely ill-looking. ‘I know I’m as ugly as a mud-fence stuck with tadpoles, but I’m not the only one.’.
[US]Western Folklore XXV:3 196: As ugly as a mud fence.
[US]Eble Campus Sl. Apr. 8: ugly as spitting/pooting in church – really bad, tasteless.

In phrases

...a buck-horse

exceptionally unattractive.

[UK]Professional Anecdotes III 194: Pope was hump-backed; Goldsmith was bandy-legged; Dr. Johnson was a Russian-bear [...] and Gibbon, the historian, was as ugly as a buck-horse.
‘Blowhard’ Jack Tench 104: She was too fat to rise without assistance, and, as they described her, ‘ugly as a buck-horse’.
...a hatful of arseholes (also ugly as a bagful of assholes, …bagful of busted arseholes, ...bagful of bums, ...bagful of busted boils, ...a bagful of monkeys, …a hatful of assholes) [arsehole n. (1)]

(Aus./N.Z./US) very ugly.

[Aus]B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 69: I once done one of me ma, but she turned out as ugly as a hatful of arseholes.
[US](con. 1950s) McAleer & Dickson Unit Pride (1981) 26: She’s uglier than a bagful of assholes.
[Aus]N. Keesing Lily on the Dustbin 25: If you had no claim to great beauty you were ‘as ugly as a hatful of arseholes.’.
[US] in Maledicta VIII 236: That girl is uglier than a hatful of assholes.
Age Sat. Extra (Melbourne) 4 Jan. 4/6: She’s as ugly as a bagful of monkeys.
[NZ]McGill Dict. of Kiwi Sl. 12/1: bagful of busted arseholes, an extreme state of ugliness, if preceded by ‘as ugly as’; or ill health, usually owing to a hangover, if preceded by ‘I feel like.’ NZA.
[UK]K. Lette Llama Parlour 97: To put it bluntly, these guys were as ugly as a bag full of bums.
[Aus]G. Seal Lingo 88: Bodily and mental afflictions, real, imagined or fervently wished, also feature strongly in Lingo. One can be [...] ugly as [...] a bagful of busted boils.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].
...a tarantula

very angry, unpleasant.

[US]A.H. Lewis Wolfville 243: But the Mexican gets ugly as a t’ran’tler at this an’ with one motion he lugs out a six-shooter.
...bull-beef

a general term of contempt; thus go to the billy-fencer and sell yourself for bull-beef.

[UK]Hotten Sl. Dict. 88: bull-beef, a term of contempt; ‘as ugly as bull-beef,’ ‘go to the billy-fencer and sell yourself for bull-beef.’.