From the Super Mario Wiki
Mort is a resident of Flopside from Super Paper Mario. He is a game lover and is always at the Flipside Arcade, from before it even opens until it closes. According to Tippi (and Tiptron), he lost more than just coins there, and also comments that he needs some video-game restraint. Originally, Mort would always lose at the games he played and would blow all his money on them. After the end of Chapter 7, though he begins to get better and after the end of the game, he ends up getting the high score on all the games there.