HN doesn't tolerate jokes well because they are usually a disruption to the conversation. If you want to make a joke here you can, but you have to ask yourself whether the joke is lame and a distraction or original and funny enough to derail the conversation momentarily. If your joke gets downvoted into oblivion that's a sign that people feel it's the former.
I generally feel that jokes on HN are acceptable if they are contained within an otherwise useful comment. If you make a comment just to make a joke that's what's not acceptable.
