Mastodon also has a different outbox format (one that's paginated), so that's the other reason their posts don't show from Lemmy. Solving the problem of following users with fake communities has also been proposed (by me, and also here), but it's generally a bad idea - a hack that will come back to bite you. If Lemmy were changed to integrate Mastodon more, it'd ideally be a big project - one that would fully embrace the idiosyncrasies of Mastodon (like muting replies and denying Follow requests) because ignoring them would lead to trouble later on.