Interesting you mention Mastodon, that uses the same protocol as Lemmy. So people on Mastodon can actually participate in Lemmy discussions (though things look and work a little different, you can often spot them because their replies to comments start with @user because that’s how it works on Mastodon). Currently Lemmy users can’t interact with Mastodon, but this isn’t because it’s not possible, just not implemented in Lemmy (yet).
Kbin has both micro-blog style (like Mastodon/Twitter) and thread style (like Lemmy/Reddit), and largely Kbin keeps them separate so they are more natural than a Mastodon user interacting with Lemmy.
But yes, we can see the Kbin communities (they call them Magazines), and they can interact with us pretty seamlessly (you know, when bugs in Kbin aren’t causing lemmy to DDOS itself). Here’s the Kbin.social meta community: lemmy.nz/c/kbinMeta@kbin.social