Conceptually, I think the way Lemmy and Mastodon would be able to interop is pretty straightforward: Each thread in either is basically just a tree of replies. They are just shown differently depending on the platform. Furthermore, Lemmy communities show up as Mastodon groups, and Lemmy threads show up as retoots from those groups, which I think is the most elegant solution.
The only issue that makes this interoperation unusable really is that Mastodon groups representing Lemmy communities just “retoot” every single comment, obliterating the TL of anyone who dares to follow those groups. Which as far as I know only happens because Mastodon refuses to be cooperative and properly follow the standards.
As for the other comments asking “why even care about this”: I think it’s worth as a long-term goal for the Fediverse to entirely separate the “view” aspect from the “content” aspect of platforms where reasonably possible, so that each user can browse all the content in their preferred platform. Not all fedi platforms need to conform to some absolute feature parity, but as I just said, there’s basically a one-to-one relationship between Lemmy and Mastodon content, so it is reasonable in this case. I’ve seen enough people here claim that they very much prefer the Lemmy format to read conversations.
Personally, my Mastodon account has different vibes from Lemmy, and for that reason alone there is a bunch of Lemmy communities I wouldn’t subscribe to, but would follow from Mastodon. The only reason why I don’t do that is because Mastodon’s side of the interop fucking sucks.