You’ve had some well-meaning but ultimately not quite accurate answers in this thread so just to clarify:
You can follow, post to and interact with Lemmy communities from Mastodon, because they’re treated the same way as a “group” on Mastodon in general.
You can NOT follow and interact with Mastodon users from Lemmy, because Mastodon accounts are individual “users” and Lemmy doesn’t have the concept of following and interacting with users, only with communities. If Lemmy ever does add a feature to let us follow other users, then in theory following Mastodon users will also become possible.