Despite the fact that he is unfunny to begin with . . . I don't think it was a very good joke. Not just because I'm atheist, either.
First of all, most atheist don't believe they'll come back as a "tree" or whatever. And secondly, he only made himself look JUST as much of a douchebag as the atheist in the joke did.
Dane Cook is just unfunny, though. If he wasn't so crazy and wild while one stage no one would like him. All his material is based on a good delivery. And is unfunny without it.