Calli, here are a couple of partial quotes (I couldn't access the one article that sounded like it was going to actually define cosmic joke--it was restricted). So these are phrases taken from Google listings.
the human condition, if faced squarely, can be viewed as a cosmic joke
The innate joy of discovering the Cosmic Joke of life
I wish I had a real understanding of what it meant (though I thank you, FNA; you helped me along quite nicely). But it sounds as though it means that we might as well be resigned to the fact that we have little control over what happens to us, and that the "joke" aspect is, as FNA said, an ironic term. Perhaps (somebody please correct me if I'm wrong) an example might be that someone who has used his hands to make a living (a violinist, a bricklayer) loses the use of them.
The point I'm trying to make is that if you, calli, have a good understanding of the concept, then perhaps you can find Chinese words or phrases that convey the same thing, without doing a literal translation.
I don't envy anybody the job of translating. Just think, for ex., of how off-putting it would be to most native English speakers if we got a literal translation of an Italian recipe for "little worms"!
EDIT: I just tried to look up another phrase, and I'll be danged if I didn't get the same site as the one above that wouldn't let me in! Is that a cosmic joke?