I think the value of the trickster is that si sees the world differently from how other people do, and so reacts differently. The trickster shares that view of the world with others purely by being, and that's why people are uncomfortable, because they don't like their comfortable certainties challenged.
But let's be honest, the trickster can be a force for both creation and destruction, which are really just two sides of the same coin called change. I do think that the Joker could be equated with an aspect of the trickster, it's just that his vision of the world and how he lives that vision is pure destruction - think about some of the more vicious and cruel things Loki did. At the end of the day the Joker's whole plan in the movie was to show that people are animals by making them tear each other apart. And I'm not so sure that he was proven wrong. Individuals proved him wrong, but as a whole he had point.
Either way, whether positive or negative, tricksters are agents of change, agents of chaos. Which is important, because societies that do not change stagnate into dead, uniform entropy and die.
Mina.