This is kind of up to the individual community, not the instance as a whole. An instance theoretically could make a general 'No memes on any community on this instance' rule but it would be awful to enforce, and it'd be easier to leave it up to communities.
That said, I think Lemmy is a long way off from having the userbase or popularity to create that problem, and the absence of karma or any analogue really narrows the impact. Personally, I've seen significantly less low-effort content here than on Reddit, with the exception of a few specific communities that exist for that purpose specifically.