I really don't like trashing Alien 3, because I've spent the past fifteen years doing that and if anything over time my opinion of it has improved, but I feel like Ripley was made bald to mask the deficiency in a lack of character arc for her. The baldness of the prisoners is a side effect of that. I just watched Alien 3 assembly cut last week, as well as the documentary that was on the blu-ray, and Sigourney says how in an exec meeting they asked Fincher how he sees Ripley in the film and he replies "Bald" with a shrug, and Sigourney laughs about that, but that really strikes me as an absence of any idea where to take the character.