Sort of, but not really? Lemmy itself doesn't really have the right feature set to follow users on Mastodon instances, and user posts don't show up, unless they're responding to posts (communities show up as users, and posts as threads/boosts), and there is no way to access an instance directly.

If you want that kind of Lemmy-Mastodon compatibility, you might want to look into something like Kbin, which is a bit more compatible with Mastodon, whilst also being rather compatible with Lemmy.