I have to say, this is one of the most confusing hypothetical questions in recent memory. I'm not even sure whether it's Heath Ledger who is my agent or the fictional character of the Joker, as created by the acting of Heath Ledger. I'm going to assume the latter, and from there I can kind of answer it.
I'm going to go with yes. Here's the thing, though I don't know exactly what adequate representation would look like, I assume the use of the word adequate rules out a lot of the worst case scenarios as having the Joker as a literary agent. For example, he can't kill me and still reasonably be considered an adequate agent. Similarly, he can't get captured by Batman and locked up in Arkham and also fulfill his role as an adequate literary agent. Also if he were to say, blow up a shitload of people and go on urban killing sprees, I doubt he'd be able to fulfill his duties as a literary agent, meeting publishers, making book deals on my behalf, negotiating contracts - that kind of thing. If he's become so notorious as to warrant widespread panic as is his proclaimed directive, then he's going to fail me as his client.
So, in the world where the Joker is an acceptable literary agent, with knowledge of the publishing industry and the capability to read and fight for my fiction, all I'd have to endure would be a number of really bizarre meetings where one of the guys at the table was dressed as a clown, asking people if they know how he got his scars, and threatening to make our pencils disappear.
It might make firing him for a better agent a little risky though...
