I don't know. I've been here for nearly a decade (I switch accounts every once in a while), and in the last six months or so, it sure feels like somethings different. Specifically, I'm noticing more joke replies that aren't downvoted. Maybe I'm just getting old.
I said the same thing a year or two ago. There's probably some truth to it, but overall it's probably just selective bias.
I became a HN regular because I saw some insightful comments backed by quality moderation. I was escaping the dumpster fire of reddit so it seemed like paradise by comparison. Over time, my usage of HN grew, and I became more exposed to the reddit-like mentality that pops up on certain topics or at certain times(i.e. weekends).
I think the quality of HN varies depending on where the more serious users are in their development schedules. It would be an interesting thing to analyze I suppose.
HN is getting to reddit's status where people make comments and everyone just upvotes because it sounds good to them.