Looking back at ancient times/medieval humanity seems to create a perception of this stale, no-fun world of pain and misery. I assume much of this is true, but I must ask, were there many forms of shitposting or other similar things? I've been to some museums in my day, and seen some stone carved genitalia, but they are always given descriptions of romantic statements and beautiful gifts to lovers. None of them say anything like, "after Gaius burned every field in the village of Saepinum, the villagers repeatedly carved him messages such as these in the relics on display": "Hey Gaius, I hope you eat this" "Gaius choke on this and go to tartarus you lazy bastard" "To Gaius: A gift" etc... The way I describe shitposting in this scenario, is things that are purely worthless comedy statements or insults, and general immaturity and stupidity.
Roman graffiti could be found around brothels and bathrooms, some of which was not only made while defecating, but also about defecating. So really not that different from reddit.
This graffiti can be washed away by the elements, but some was found preserved quite well at Pompeii:
Vesuvius dumped ashes and lapilli on Pompeii for 36 hours, sealing the entire city up to an average height of 20 feet. Since the 18th century, archaeologists have excavated about two-thirds, including some 109 acres of public buildings, stores and homes. The city’s well-preserved first level has given archaeologists, historians and classicists an unparalleled view of the ancient world, brought to a halt in the middle of an ordinary day.
From the very beginning, archaeologists noticed copious amounts of graffiti on the outsides of buildings. In the late 1800s, scholars began making careful copies of Latin inscriptions throughout the ancient Roman world, including Pompeii, and cataloging them. This effort is a boon to scholars like Benefiel, since more than 90 percent of Pompeii’s recorded graffiti have since been erased by exposure to the elements.
Source: https://www.smithsonianmag.com/history/reading-the-writing-on-pompeiis-walls-1969367/
Searchable list: http://agp.wlu.edu/Graffiti/results?browse=true
A few of my favorites from this Source (nsfw?):
II.7 (gladiator barracks); 8767: Floronius, privileged soldier of the 7th legion, was here. The women did not know of his presence. Only six women came to know, too few for such a stallion
III.5.1 (House of Pascius Hermes; left of the door); 7716: To the one defecating here. Beware of the curse. If you look down on this curse, may you have an angry Jupiter for an enemy
IX.8.3 (House of the Centenary; in the latrine near the front door); 5243: “Secundus defecated here” three time on one wall
While they may not directly answer your question, you may be interested in the following:
Medieval Jesters - What kind of jokes did they tell? as answered by u/texpeare and others
[What sort of humour was popular in the late Middle Ages?] (https://www.reddit.com/r/AskHistorians/comments/81xno7/what_sort_of_humour_was_popular_in_the_late/) by u/Lewed_Losel and others
Due to tagging limitations, please see the next comment for further links.
Thanks everyone! this has been quite insightful.