Consider this argument:
- Finding something funny always involves being surprised.
- An omniscient being is never surprised.
- So, an omniscient being finds nothing funny.
Premise (1) seems false: Consider the phenomenon of the person who is bad at telling jokes, because the closer he gets to the punchline, the harder it is for him to keep from laughing (that's me!). Such an annoying person obviously finds the joke funny, even if he's told it many times and it appears is not at all surprised.
Some sort of thwarting of prima facie reasonable expectations may be an essential feature of finding something funny, but such thwarting can happen without surprise, and could well happen eternally and unchangingly.
As for premise (2), I am not sure. First of all, suppose that x became omniscient at time t1. Then he could well be surprised at t1—for instance, surprised by becoming omniscient! But we should charitably understand "omniscient" in the argument as "eternally omniscient" (either omnitemporally eternally or timelessly eternally). Even so, it is not obvious to me that someone couldn't be eternally surprised at something. (One might think that being always surprised at, say, the beauty of the world or the possibility of evildoing is a good feature of a human being.)
Interestingly, I don't know if I get to beat up on both premises (1) and (2). For if there is such a possibility as eternal surprise, then maybe the person who can't stop laughing while telling a joke is simply always surprised by it. Hence, it may be that one of (1) and (2) is true. But at most one, and the argument needs both.
There may be other arguments why an omniscient being couldn't find anything funny. But the one I started with fails.