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.
Both formats can serve a purpose, I haven’t put enough/any effort into my mastodon, but the intention was for it to replace bluesky for me, where I tend to have more conversations in threads, than I do here on lemmy.