I recall overhearing that question way too often on Halloween. Someone sees a costume and 'doesn't get it'.
A few dozen people at work are dressed up this year. Today, I'm pretending to be an extra on a movie set. I'm not wearing anything that stands out. It suits me well.
I recall as a kid, I made up my own superhero outfit (of course it had a cape) as so of course I got that whole, "Who are you supposed to be?'
In my more professional years when the whole 'office' dressed up, I again 'created' my own Halloween concepts. One year I was "late to work" – mismatched shoes and socks, half tucked in shirt, mis-buttoned shirt, half done tie, and of course my hair was messed up. Then I went to the grocery store to get donuts for the morning and got some strange looks. I'm sure I overheard someone ask, "Who is he supposed to be?"
The following year I was "Thad" my evil-twin brother. "Thad" had longer hair and wore his collar up and spoke with a really bad accent. I can't recall where he was even from. I of course got the question, "Who are you supposed to be?" – so maybe I was over-hearing "who are you supposed to be?" being asked about me all those years.