The Collaborative International Dictionary of English v.0.48:

mutt \mutt\ n.
   An dog that is of inferior quality or of mixed breed.

   Syn: cur, mongrel.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mutt
    n 1: an inferior dog or one of mixed breed [syn: cur,
         mongrel, mutt]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "mutt":
   asshole, blockhead, boob, booby, chump, cur, dimwit, ding-a-ling,
   dingbat, dingdong, dolt, dope, dumbbell, galoot, goof, idiot,
   ignoramus, jerk, jerk-off, klutz, lurcher, mongrel, numskull,
   pariah dog, prize sap, sap, saphead, sawney, schlemiel