I don't think screenshots really fit the criteria for qualifying as a "meme"—humorous or not.
I understand why screenshots feel meme-adjacent. They can be funny and shareable; however, I think memes (as we've come to be most used to the term today) are chiefly expressed through image panels containing text that succinctly convey humour. Screenshots rarely fit this criteria because while sometimes funny, they often lack the visual aspect that makes "memes"... memes.
Think about the distinction between 'xkcd's' and 4chan screenshots for example. We often come across the phrase "insert xkcd about 'x' topic here" but we can't do the same for 4chan or Xitter screenshots. This is what i think meme communities should represent the most: image panels that succinctly convey humour and can be (and often are) deployed at a moment's notice to satisfy a humourous intent.
So yes, i think we need a rule banning screenshot posts because they don't fit the vibe of typical memes (although i get this is subjective) and there are already numerous communities for those kinds of posts.