If HN is trying to achieve a soulless takes-itself-way-too-seriously environment, then I'd agree with you. Hopefully that's not the case (but probably is, per the observations I've accumulated over the years of hanging around here).
> intellectually-stimulating conversation
Humor can be intellectually-stimulating, even in the form of clever puns. Whether or not the pun in question was clever (or at least sufficiently-clever to be amusing) is a different question.
> diverse group of people that have diverse (and sometimes opposing) viewpoints.
Humor happens to be an excellent way to both convey and consume those viewpoints, especially in a way that's actually conducive to all parties actually coming away with a better understanding of one another.
It's a housekeeping/ broken windows thing, puns and off topic one liners that exist to solicit some type of thoughtless reaction in the reader, are the first signs of a forum in decline.
> It's not that funny stuff isn't amusing, it's that it's counter to what HN might be trying to achieve.
So, keep it serious. No Having fun. Gotchya.