On the other hand, in forums where people upvote humour, there can be mechanisms that actually are counterproductive. E.g. on Reddit where there is something serious that you are interested in, but some idiot starts a punning thread which takes off and runs for pages. Or slashdot, where a witty one line (simplistic) put down off a complex issue will recieve maximum karma, whereas a well thought out retort will struggle to break the readability threshold.
Hacker News has a number of mechanisms for impeding flame-wars, and I think some of them also impede the flow of humour. No fast replies for instance slows things down, and also the depth algorithm because PG doesn't like deep threads. Another one is de-emphasising karma. If people hand out karma for being entertained, then the system 'rewards' you for being entertaining, so again people go for the slashdot style one-off witty put down.
On the gripping hand, now that HN de-emphsises karma (and has weird rules about what can and cannot be down-voted), I'm much less likely to actually bother. Karma is already pointless, but rubbing our faces in it simply reduces participation. The ideal system would 'reward' well thought out replies, and I don't see the current system doing that anytime soon.
On the invisible hand humour is an important part of geek culture, as are references to things such as invisible hands and gripping hands. I'm not sure why that is, but word-play and recursion (important parts of being a really clever programmer) are already pretty close to humour, or possibly even forming some of the foundations of it.
Which leads me to conjecture that all of the really good programmers have a healthy sense of humour, and that implies that if hacker news is overrun with people who don't have a good sense of humour, that would indicate the ratio of good programmers here is falling. Why would that be? Perhaps because of the emphasis on money, and winning the startup lottery. The programmers who are only in it for the money tend to view themselves as more 'serious' - so I suspect they are the source of the "humourless git" demographic.