As a recent arrival who has been trying to figure out what the community standard is, I have been paying close attention to this question. In line with rms's comment, no, I don't think humour gets downvoted per se. Cheap shots definitely are, which covers a lot of sarcasm, as are quips with no additional content and general rudeness. What I would say is that comments which lack content get punished even more when attempting (either successfully or not) to get a laugh.

I certainly don't think there is anything wrong with having a forum where every line doesn't play to the peanut gallery.

I think you're on the right track. In my experience, funny is not downvoted for funny's sake. However, there's a strong correlation between funny comments and short/low quality/insubstantial comments, and the latter will get downvoted.

If you want to be funny, be interesting at the same time. If you can't, then save the joke for reddit.

I think there's a bias against funny and amusing comments, and I think it's appropriate. The internet is an inexhaustible source of mildly amusing, even fairly funny stuff. There's a limited amount of room on the HN front page, and it shouldn't be wasted on humor of the quality that can be found in vast quantities all over the internet.

Another thing that differentiates humor on HN from humor elsewhere is that disrespectful commentary, no matter the level of wit, is rejected.