It's not that everyone's serious, it's that this community's greatest fear is that HN will get overrun by frivolous comments and other weeds. As a result, one of the tradeoffs that's evolved is that people reflexively downvote jokes. (Non-funny jokes get hammered particularly hard.) It's best not to take it personally and work on one's noise/signal ratio.
I've mostly avoided trying to be funny on HN, but I've been voted up for cracking wise on a few occasions, and have voted up things that made me genuinely laugh.
It's a dreary business trying to analyze humor, even more so to analyze what kind of humor plays well at Hacker News, but I'd say the more original the joke, and the more the joke makes you think afterward, the better it will do.
As an aside, my goodness, this is such an HN comment.
