I've noticed more new users making poor comments, but I feel like I've also seen fewer joke/one-liner comments at the top. I think it's more an influx of new users than just a decrease in quality. Perhaps the guidelines should be made more visible, and updated some. For one, the community norms about humor/snark aren't actually in there (from what I recall), but they're an important part of how to comment on HN.

Edit: Hmm, that gives me an idea. Perhaps give new users a few "sample comments" of what's good, and a few of which are bad. IE: "This comment by grellas comes from an informed background, and is free of snark despite contesting some of the claims in the article. Goofus sees the same article, and decides he can make a good pun on the title, and throws in an insult to the author based on something he sort of remembers hearing about a while ago. Be a grellas, not a goofus!" Well, perhaps less hokey, but you get the idea.