The Collaborative International Dictionary of English v.0.48:

Unbeautiful \Unbeautiful\
   See beautiful.

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "unbeautiful":
   beautiless, blemished, blotted, cacophonic, cacophonous, defaced,
   disfigured, dysphemistic, dysphemized, hideous, homely,
   ill-favored, ill-looking, inelegant, marred, plain, short on looks,
   spoiled, uglified, ugly, ugly as hell, ugly as sin, unaesthetic,
   unalluring, unattractive, unbeauteous, uncomely, unhandsome,
   unlovely, unpleasing, unpretty, unsightly