It earns a place in this list not because it is usually harmful — it usually is not — but because of what it can do at volume and at the wrong moment: make a discussion unusable without anyone having to argue against it.
Why it works as a tactic, when it is one
Refuting an argument is work. Making a thread not worth reading is almost free, and it does not expose the person doing it to any counter-argument, because none was made.
It is also cheap to defend. Since nothing was asserted, there is nothing to retract, and objecting can be answered with "it was a joke" — which puts the burden of seeming humourless on you rather than on the post.
Recognising the tactical version
The distinguishing feature is timing rather than content:
- It arrives when a conversation was going somewhere, not at random
- It is aimed at the thread rather than at any participant
- Objections are met with "it's just a joke", never with an argument
- The same account does it repeatedly in the same place
Every one of these has an innocent reading. People are funny when they are bored, threads do wander, and someone who jokes a lot in one community is usually just a member of it. This is the weakest signal set on the site and should be held loosely.
What to do
Ignoring it is usually right, and this is the case the folk advice fits best — an untargeted post, with no particular audience for your reply, in a place where saying nothing costs you nothing. That is a narrower set of conditions than the advice is normally offered under, and it is worth noticing when they hold.
If it is persistent and aimed at a particular space, that is a moderation question rather than a conversational one, and the people who run the space are better placed than you to answer it.