Low-content jokes never did particularly well on HN. Like it or loathe it, it's been a cultural feature here for years.
I don't remember it being this bad maybe ~8 years ago.
yeah, the HN commenter majority are stuffy and humourless but quick on the downvote. just leave them to their playground.
It has nothing to do with that. The fear is not what's added, but what's removed. Experience tells us that jokes will quickly dominate the top of threads, leading to a loss for serious comments. Sure, there's enough room for as many comments as you like in a thread, but only the top x actually get read, usually.
FWIW, I thought the comment was pretty funny, and the first answer was even better.