Humour is generally downvoted on HN, because us programmers (mostly) take things too literally. But, seriously, tech really is the biggest laugh because we take each generation of software, language, platforms so seriously, completely ignoring the fact that we are just glorified typists trying to find patterns where none might exist even without having the necessary background to do so.
No, humor is generally downvoted on HN because most of it is lazy, and no one wants HN to turn into another Reddit. Lazy humor is a Reddit staple, and there's already far too much Reddit around as it is. Good humor tends to fare reasonably well, and considerable allowance is made for attempts which fail to be funny but still show effort - otherwise, I doubt I'd get away with doggerel in rhyming couplets  and similar such excesses.
I also see some pretty good, very nuanced jokes (the kind you chuckle about at first, then the second level of the joke hits and you chuckle a little harder) getting voted down heavily as well.
Dunno if I can buy the "most of it is lazy" line here.
We're just going to downvote jokes-even if one is made completely within context of discussion? Even if it's an absurdist take on a topic that actually manages to get a laugh-while also / potentially bringing up a valid point?
I'm not asking to argue this with you specifically, I'm just kind of curious in general where that line is drawn with the community.
One thing to keep in mind is that if there's good humor in a comment that has good substantive content as well, it's likely not a bad thing and won't be downvoted. It's comments that are posted for (mostly) humor value alone that I think many in the community are trying to avoid.
Edit to add:
I want to be clear that I'm not anti-humor in general. I do highly value having a place where more serious discussion can be had. And I'm glad HN tends to be that kind of place. There are other places in the world (some of them even on the internet :) where more jovial times can be had.
But alas, I'm fighting windmills here-this is a subject I've discussed in other threads; I understand the want for the community to remain at a certain level. "Jokes"/"Humor" however seem like boogeymen/scapegoats to that end; at least in my opinion.
Edit: Caught your edit here-
One thing to keep in mind is that if there's good human in a comment that has good substantive content as well, it's likely not a bad thing and won't be downvoted.
One would think. I've definitely observed the exact opposite with a frequency that's hard to put aside as 'outlier'. But I get your point.
So far it seems that humour that makes an effort or that is funny doesn't get downvoted and might get upvotes; other humour is ignored or downvoted.
I think jokes get one or two downvotes, just enough to turn them grey, and then people stop downvoting. But maybe I'm wrong about that.
Also, a joke subthread can often have a sort of "gravitational force" that engages people in replying to it such that they never end up contributing to the "real" discussion happening below it, because they never make it there. This effect gets worse the earlier in the comments the joke appears. (This also happens with political tangent-subthreads, but nobody likes those.)
I think (these) people would not be made nearly as upset by jokes if they didn't "interrupt" the flow of conversation by appearing first in comment-subthread-ranking. I'm not sure how that could be done without manual moderation, though; people will always want to reward jokes with points, because jokes do have genuine utility to their consumers. They just can choke out the production of more serious worthwhile sibling contributions.