What is the IQ of someone with microcephaly?

Real low. Not seen outside of institutions, unless you include todd browning's classic ''freaks''.
Decreased. Microcephaly regardless of cause has a very strong association with decreased iq, neurodevelopmental delay and learning disabilities.
Rare. True microcephaly (head circumference in the 0.1 percentile, roughly) is rare. A small head secondary to e.g., fetal alcohol syndrome is not so rare (and not nearly so severe). True microcephalics have very severe cognitive deficits so iq measurement is meaningless (it fails at both extremes). With fas, the average iq is about 60.