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 ever consider choosing a non-Mastodon Fediverse software, I’d recommend Wafrn as it implements Bluesky’s protocol alongside ActivityPub meaning that it can interact with Bsky without a need for a bridge. A nice way to keep your follows without fully succumbing to the shitty moderators of Bsky.