1. joke quality is how much sense the punchline makes as a completion of the setup minus the probability of finding the punchline given just the setup
2. jokes are subjective, people disagree wildly about how good a joke is, so if you aggregate preferences you get mediocre jokes at best.
I think 1 is the important part. If it was just 2 - comedians with widespread appeal wouldn't exist, but they do.
But 1 is basically the exact opposite of how LLM work. So it's no surprise it's hard for them.
It would be interesting to compare how well LLMs can estimate how funny a joke is vs how good they are at generating jokes.