Do you think it would be okay if I use my roommate's command hook to hang up my towel in the bathroom?
Is there a reason that you want to use his command hook instead of a towel rack?
I don't want to use a normal towel hanger because its common. I want to be more extravagant.
Wanting to use the hook instead of the towel rack sounds pretty reasonable if it has more than one hook.  It would be considerate to ask your roommate first though, since they installed it.
Of course, it has more than one hook.
If he installed one that has multiple hooks it was likely for both of you to use anyway. It would still be a good idea to ask  them about using it first though.