At the moment, it is not possible to subscribe to user accounts on Lemmy, only communities. This means it is basically not possible monitor anything taking place on Mastodon from Lemmy because communities don't exist on Mastodon. Mastodon users are able to follow Lemmy users (though not communities), and can reply to posts / comments they make. These replies are visible on Lemmy. From the Mastodon user's perspective, there is no difference between a Lemmy post or a comment though. This may change as things develop.