My experience has been that Monty Python and Douglas Adams references are the clear exceptions to the no humor rules on HN, though I don't suppose you'll find that written down anywhere.

From what I've seen, a bit of reasonably mature humor seems acceptable, it's just multiple posts devoted to jokes derailing the conversation where it's frowned upon. I quite like this system, as it prevents Reddit-esque threads with the same tired memes being repeated back and forth ad nauseum, but still allows for a bit of levity to lighten the conversation and provide some mental refreshment.

And yes, I realize I just called a Monty Python and the Holy Grail reference mature.

So long as you define 'mature' to mean 'in line with the average HN reader's ego and self-image', you're pretty much on point.

That may be the case, but I appreciate it nonetheless - most posters are pretty good at avoiding jokes at the OP's expense, for example, which contributes to HN's relatively good community.