The Collaborative International Dictionary of English v.0.48:

mismatched \mis*matched"\ adj.
   1. Matched unsuitably; grouped into pairs or sets with
      incompatible characteristics. [Narrower terms:
      ill-sorted, incompatible, mismated, unsuited; odd,
      unmatched, unmated, unpaired ] Also See: {incompatible.
      Antonym: matched.
      [WordNet 1.5]

   2. Of very unequal ability; -- of contestants in a contest.
      The less able contestant is said to be overmatched or
      outclassed.

   Syn: uneven, not fairly matched.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mismatched
    adj 1: either not matched or unsuitably matched [ant: matched]
    2: (of a contest or contestants) not fairly matched as
       opponents; "vaudeville...waged an uneven battle against the
       church" [syn: mismatched, uneven]