Dimentio always wears a smile, even in his final curtain call when he is by Mario, Bowser, Peach, and the Pure Hearts. He often speaks in metaphors ("Now we must duel, like two gleaming banjos on a moonlit stoop!"), and plots throughout the game to overthrow Count Bleck, though at first his intentions are subtle at best. He controls dimensions, and has one of his own creation called Dimension D which makes him precisely 256 more times more powerful when he is in it. He is also thought to have written the Dark Prognosticus.
Dimentio was most likely destroyed, although the theatrical charlatan may indeed return in future games to haunt Mario and Luigi.