Yet far less popular and well known than any other 3rd party char. He's the first char that, at the time of inclusion, A) only had one game, that B) didn't appear on a Nintendo system, and C) is current-gen and thus currently weakens Nintendo's relative line-up of games.Atlus isn't buying ad space. Joker's inclusion wasn't a ploy by Atlus or Nintendo to make Persona more popular. They put a popular character into Smash cause he was popular.
But sure, maybe they just wanted to do a game char that is popular and fresh at the time.