I am quite happy to have a Google search phrase that brings up one of my posts: weaselverse darkest timeline. It's niche but still...
My understanding is that Lemmy's cross-posting between instances makes Google think they are just a "farm" of sites harvesting clicks and ad revenue. That will really impact their ranking or even if they get indexed at all.
I hope it's only a matter of time before Google adjust their algorithms to take account of the Fediverse but I am not holding my breath. I have pondered how to improve Lemmy's SEO but I am drawing a blank.