Technically speaking, it is illegal to have anything hanging from your rearview mirror as you drive. Everything, including parking permits, are considered line of sight impairments. You are supposed to actually remove parking tags from your mirror when you go to drive. Cops won't pull you over for it as it's a pretty minor offense. But, if they pull you over for something else and want to be arses about it, they can tack that on top of the rest of your ticket.