While I agree that seems the case, there likely has been a corresponding shift in niche communities, even if the effect is less measurable. I don’t check in on my niche subreddits often anymore (I’ve fully moved over to here and Discord), but with some of them, their quality has dipped slightly. Possibly could be chalked up to continuing trends independent of the migration, though.
Smaller communities also require builders, so I’m sure the necessity alone has driven content quality on Lemmy.