It’s not a perfect fit, so if you want to use both Lemmy and Mastodon I strongly advice you to have an account in each.
That said, you can follow Lemmy users or communities on Mastodon, likes made on Mastodon to Lemmy posts equate to upvotes here. If you reply to a comment sent from Mastodon, the Mastodon user will see it. But to fully enjoy both, you need 2 accounts.