So, you won't see Mastodon content on Lemmy unless a Mastodon user has posted in a group (the generic term for community, subreddits, etc). For example, here's an exchange I had with some Mastodon users. Groups don't always come from Lemmy and as a Lemmy user you can subscribe to more Mastodon centric groups like !histodons@a.gup.pe or even PeerTube channels like !veronicaexplains_channel@tilvids.com. Direct user-to-user microblog style interaction with Mastodon users is not supported, and that's mostly a design choice of the devs. Projects like kbin/mbin seek to bridge the gap and directly support both experiences.