Lemmy and Kbin should be pretty interoperable, but Lemmy and Mastodon don't federate particularly well, or at least, the user experience of federation is not great. The formats are just very different since Mastodon doesn't have much of a concept of groups or communities and doesn't have post titles and so on.

But you can follow Lemmy communities from Mastodon and posts to the community will show up in your Mastodon feed, and you can boost, favorite (upvote), and comment on the post.