The Collaborative International Dictionary of English v.0.48:

muttonhead \muttonhead\ n.
   A stupid or foolish person; a dolt; a numbskull; a blockhead;
   -- disparaging and offensive. These words are used to express
   a low opinion of someone's intelligence. [disparaging]

   Syn: dunce, dunderhead, numskull, blockhead, bonehead,
        lunkhead, hammerhead, knucklehead, loggerhead, shithead,
        fuckhead.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

muttonhead
    n 1: a stupid person; these words are used to express a low
         opinion of someone's intelligence [syn: dunce,
         dunderhead, numskull, blockhead, bonehead,
         lunkhead, hammerhead, knucklehead, loggerhead,
         muttonhead, shithead, dumbass, fuckhead]