Or the result of a silly bet.
There's a lot of background on the little details of the TOS movies, especially ST:TMP which was grossly overthought and really represented something of a 90/10 split of material left out and material included in the actual film. We aren't really obligated to believe all of it, though, not if we confess to the canon faith. In onscreen terms, Ilia's baldness is not explained as being species-typical, species-atypical or anything else. Her oath of celibacy is not extrapolated on, either - perhaps she was a notorious sex offender who was all over the news before she reformed and joined Starfleet?