If you mean lemmy.world, Lemmy doesn't display microblog posts unless they're posted to a community, which is a confusing process. Threads isn't fully federating right now anyway though. Even if you're on Mastodon, you'll only be a able to see posts from specific Threads accounts who have had federation enabled, which is mostly just a few engineers iirc.

Threads isn’t fully federated, meaning currently there aren’t a lot of Thread posts on the Fediverse. Also, since Lemmy isn’t like Threads (compared to mastodon), you won’t really see any posts. Overall Threads is mostly a Mastodon problem (though lemmy is also effected) and you probably won’t experience a big difference