You could try #shitpost or #shitposting but I don't see those commonly used.
There are two main instances where shitposters congregate: beige.party and jorts.horse
Beige is newer, bigger, and more well known. Jorts is unapologetically weird and absurdist. I love them both TBH.
You can browse the "public local timeline" of Beige here:
https://beige.party/public/local
That is all public posts from users on that server in reverse chronological order.
I can't get the same thing to work for Jorts.
If you use an app that lets you browse other instances' local timelines (like Fedilab) you can subscribe to posters in those timelines more easily then the web interface's copy-paste-search dance. (For the record I can browse Jorts' local timeline in Fedilab)
The easiest way to access the local timeline is by just creating an account on a server, which should work just fine too.