The Joker is, in his own mad way, trying to wake people up. His insanity is an understandable response to the corruption of Gotham City. The Joker has seen what happens when people go along, when police take bribes, when conformity and greed take over, and has responded with anarchy. He has a message that isn't "I want more money now," and that's why he's "a better class of criminal." He's happy to burn the money because he's crusading for what he sees as the truth--that the world is insane, life is pointless and you may as well laugh. He sees humans as people scrabbling for nickels while their houses burn down, and he's gluing quarters to the sidewalk to watch what happens. Perhaps he's right.