If a mastodon user mentions (@) a community, they’ll post there. They can also reply to comments on Lemmy and you’ll see it. Unfortunately I don’t think Lemmy has a way to see regular mastodon accounts/comments right now other than that.
@Lazylazycat as an example, hello! i’m replying from Mastodon on https://mastodon.social.
for some reason i can’t see your profile pictures (on lemmy accounts, kbin are fine apparently)
Oh hi! See on my Lemmy app you just look like any other user, that’s cool. I don’t have a profile pic but maybe you wouldn’t see it if I did?