As far as I can tell from my venture into Mastodon’s feeds, the demographics of that part of Fediverse isn’t that different from what Lemmy has: the same political takes, FOSS stuff, “big corpos bad”, etc.
I wonder, is there anything that Mastodon excels at that Lemmy just doesn’t provide? For example, I noticed that there are game developers and artists posting their stuff on Mastodon. Not many game developers (or any creatives, really) on Lemmy though.
If you use Mbin, or I believe Piefed now too, instead of Lemmy, you can interact with Mastodon freely without having to get a separate account there. I’m subscribed to a lot of Mastodon accounts as a threadiverse user, and I only have this one account.