It's easy to pass judgement based on some categorical expectation that service staff (whether airline, hotel, restaurant, etc.) should always maintain a professional distance from those they are serving.

However, none of us here (except perhaps the OP) knows the actual interpersonal dynamic between the FA and the pax. If the pax is being warm, friendly, and agreeable to further conversation, it may be perfectly fine for the FA to ask for an autograph. As far as we know, the FA could have served this pax in the past. Maybe they are bonding over some shared interest. Maybe their kids both play in the same tee-ball league. Maybe the pax is hitting on the FA and she is playing right to his hand. I think minding one's own beeswax is the appropriate course of action here.