WordNet (r) 3.0 (2006):

unaesthetic
    adj 1: violating aesthetic canons or requirements; deficient in
           tastefulness or beauty; "inaesthetic and quite
           unintellectual"; "peered through those inaesthetic
           spectacles" [syn: inaesthetic, unaesthetic] [ant:
           aesthetic, aesthetical, esthetic, esthetical]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "unaesthetic":
   beautiless, blemished, blotted, cacophonic, cacophonous, defaced,
   disfigured, dysphemistic, dysphemized, homely, inelegant, marred,
   plain, short on looks, spoiled, uglified, ugly, ugly as hell,
   ugly as sin, unartistic, unattractive, unbeautiful, uncomely,
   undecorative, unhandsome, unlovely, unornamental, unpleasing,
   unpretty, unsightly