Right now there is a bit left to be desired when it comes to lemmys accessibility features, but it’s a good idea to be mindful of the fact the fediverse and its platforms tends to have pretty universal accessibility features that will likely come to lemmy sooner rather than later
Link to the knowyourmeme.com page, maybe?
Alternatively, for a lot of meme formats it would be appropriate to use the text embedded in the image as the alt text.