I think it depends solely on one thing: Does it add to the topic at hand in a way that promotes intelligent conversation? Lazy or dismissive humour doesn't. It

I think it depends solely on one thing: Does it add to the topic at hand in a way that promotes intelligent conversation? Lazy or dismissive humour doesn't. It usually is a stopping point, or results in a thread of follow-up jokes deviating further and further from the actual conversations, each one trying to out-clever the previous one - a distraction far away from what was posted and intended. That's what usually gets downvoted or outright rejected on HN.