At the moment, you can't subsribe to individual users on Lemmy, so there's no way to really consume posts from Mastodon, but I am pretty sure if a Mastodon user replies to a Lemmy comment, it will be visible on Lemmy. Problem is, as Knighthawk points out, Lemmy threads are kinda unintelligible on Mastodon. I suspect this will improve with time.
Yeah it would be cool to be able to only have it show lemmy posts and not comments in your timeline, but that makes sense it's not there yet. That seems like it would be a tall order development-wise