If you really don’t get the “least/most adjective category” meme, I will explain - and therefore ruin - the joke:
Let’s imagine a category of people, an eminent trait of said category, and an element exhibiting it. For example the Italians are known to be fanatical about food orthodoxy, and you spot online an Italian guy raging about the wrong shape of pasta being used for carbonara.
You comment: [this guy here is the] “least fanatic Italian”, implying that as fanatical as this guy is, the others are worse.
What have you achieved? With only three words you have insulted his category (Italians are a bunch of fanatics) and the guy himself, by reducing him to a stereotype. Two offenses in three words, an excellent ratio. Just to be clear though, most of the time these are not used to mean the insult in a really insulting manner - it’s more of a banter.
It’s a pretty common way to insult people online, I’m surprised you haven’t encountered it much. Hope I helped.