From the guidelines (http://ycombinator.com/newsguidelines.html): "Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon."

From what I can tell, jokes are not tolerated here because they lower the signal-to-noise ratio. It's not that HN voters are humorless, it's just that they don't want HN to be a place people come for humor, because there's plenty of other spaces on the web for that. I'm starting to think of it as a college discussion group - you go to learn, not to hear Simpsons jokes.

Also the "overlords" joke has a long and tired memetic history in the hackersphere, which would explain the quantity of downvotes.