Green’s Dictionary of Slang

butter adj.1

also butters
[butt n.1 (1a) + ? play on cheesy adj.2 (4); ? note bhutto n.]

1. (orig. US campus) of an object or person, unfashionable, unsophisticated; also as n.

[US]P. Munro Sl. U. 51: butter not cool, nerdy.
[US]R.C. Cruz Straight Outta Compton 77: Hey, man, that’s real butter!
Skins ser.1 ep.3 [TV script] Hey, buttas, why you messin’ my flow?

2. of a woman, unattractive.

[UK]Guardian G2 3 Aug. 3: The kids says that if you is butters you is ugly.
[US]BlazinParadise ‘Blazing Squad Language’ [Internet] Butters – A ugly thing or person.
theculturetrip.com ‘Guide to London Slang 10 Jan. [Internet] Butters – ugly as hell.