I'm not sure how to phrase this well but I feel like you're giving memes way too much credit by comparing them to idioms. There is nothing being added to the message when paired with a generic meme template so it's not as much a method to express yourself as it is an attempt at giving the reader a dopamine hit when they recognize the image. It's also different than words or phrases evolving because it's the image template, not the words, giving the dopamine hit.

I think my issue with them can be described as this: Most memes aren't made to communicate an idea, they're made to get attention. They're the visual equivalent of a clickbait headline and I don't think they're adding to or changing our language. If what you have to say is too banal to be said without an image template, why make it if not for those sweet sweet upvotes?

I'm not necessarily arguing here, just trying to find the right words to express my feelings on it.