I don't think DMC is suitable for "official" tournaments outside of fan organization. It has no actual competitive value because it's a single player game.
Sure you can judge the most stylish action but that's way too subjective and still is not a competitive thing. Maybe something about who scores the most Stylish Points is all I can think of that can come close to making it a legit competition. Kinda like it is for say, I dunno, Tetris.