You got to follow a bunch of people before it starts to feel worthwhile, see if anybody you know from outside of Mastodon has a profile there, or open up different instances and check out people there.

If you want you can even follow people and Communities (though on Mastodon they’re called groups) from Lemmy and reply to them from Mastodon (it is a little bit clunky).

Though for best results you should try and follow people on Mastodon or similar federated microblogging platforms, since people who they follow also have a chance of showing up in your feed.