Two possibilities come to mind:
1) Some women (like Mitena Haro and the background trade official) do grow hair. And male Bolians like Mot enter this profession for their benefit. Maybe there's even an expectation for male barbers on Bolarus.
2) Even if everyone on Bolarus is bald, they've been dealing with other races for centuries. The several (at least three) wigged Bolians show an appreciation of hair, at least, so it's not out of the question that this fascination is cultural, and Mot (and V'Sal, and the two other Bolian barbers seen in TNG) gravitated towards this as a career to escape the duldrums of their homeworld.