Roy is a "more filling" meal than Haley (3 good mental stats rather than 2). And we know Haley's INT is above average - she has "bonus languages" according to Elan.
If she has good Charisma and good Int, then, to be a noticeably worse meal than Roy "Tasty, but too sweet"
then maybe her worst mental stat is Wisdom?
This thread:
http://www.giantitp.com/forums/showt...yer-Taste-buds
discussed the potential meanings behind the mind flayer's reaction to each person.
An alternate take, coming to a similar conclusion: