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]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "mismatched":
   asymmetric, discordant, disparate, disproportionate, ill-adapted,
   ill-assorted, ill-chosen, ill-fitted, ill-matched, ill-sorted,
   ill-suited, ill-timed, improper, inadequate, inadmissible,
   inapplicable, inapposite, inappropriate, inapt, incompatible,
   incongruous, inconsistent, inept, infelicitous, inharmonious,
   insufficient, irregular, irrelevant, mal a propos, maladjusted,
   malapropos, misjoined, mismated, misplaced, odd, out of character,
   out of joint, out of keeping, out of line, out of place,
   out of proportion, out of season, out of time, out of tune, skew,
   skewed, unadapted, unapt, unbecoming, unbefitting, uncongenial,
   unequal, uneven, unfit, unfitted, unfitting, unqualified,
   unseasonable, unseemly, unsuitable, unsuited, untimely