It seems it is possible to follow a Lemmy user from a Mastodon account. Not sure how it might work the other way around. Mastodon, of course, uses a “user follows other users” model, whereas Lemmy is slightly more “forum like”.

It is also possible to follow a Lemmy “community” from a Mastodon account - but I haven’t quite sussed how the posts and comments might appear.

But I don’t think any Lemmy posts turn up on the Mastodon home timeline. It looks like you have to select the relevant user/community from your own “following” list, to read posts.