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.
I will say that boosts can make posts spread on Mastodon a bit more quickly than I’ve seen Lemmy posts spread on Lemmy. But Lemmy posts aren’t particularly Mastodon-friendly, I do wish they were formatted to be a bit more readable.