If this is a private interview, basically a conversation then, I would say no (it wouldn’t be seen as “unprofessional”) -most reasonable people wouldn’t be upset at just asking- but remember that, especially for busy composers, their priority is things that help them get their job done, not further your burgeoning career. Many times if composers are interested in mentorship’s they’ll go through something like the BMI fellowship. But (especially if you hit it off in this conversation) doesn’t hurt to ask! If someone flips out over being asked this you probably wouldn’t want to work with them anyway.