Mastodon is connected to Lemmy only in such a way that Mastodon posts are interoperable with Lemmy posts and vice versa, but most clients (even Mastodon’s web app) don’t support all Lemmy functions.
What this means in reality is that you can subscribe to Lemmy communities or even Lemmy users on a Mastodon client by “following” them, but that’s mostly it. Support may vary by client.
In my Mastodon client, I can see communities like they are users, but I can’t see any posts. Technically they are interoperable, but the client doesn’t support viewing Lemmy posts.
It would be very complicated for a single client to support all platforms that use ActivityPub. So this is likely the way it’ll be.
TLDR: just use a Lemmy account for Lemmy. There aren’t a whole lot of non-techie reasons right now to use a Mastodon account to interact with Lemmy.