Mastodon supports long form and rich content perfectly fine. The problem is that Lemmy and Kbin extend ActivityPub in a way that nothing else does. While most of the Fediverse uses Note objects, Lemmy chose to use Page objects. Mastodon supports Pages but only renders a title and a link because it doesn’t really know what pages are.
Pages represent web pages, whereas notes represent “a short written work typically less than a single paragraph in length”. In my opinion, using Page was a mistake on Lemmy’s end. Just like Lemmy won’t support Place objects, I’m not sure if any other platform will ever support Page objects, because Pages are much bigger in scope than anything most Fediverse applications ever deal with.
There are also other problems. Lemmy expects the community to be CC’ed or Federation may break.
Something that’ll undoubtably confuse people is that Lemmy will send a Create to create a post (makes sense) followed by a boost (Announce) to populate it across servers. In Mastodon, this manifests as a long list of boosted posts. This is the only way Lemmy can spread comments to every other server, but it’ll flood any normal timeline with boosts.
Notation invented by other platforms (!community) isn’t going to make it into Mastodon, I don’t think. You can just paste a full URL (lemmy.world/c/linuxmemes) into Mastodon and get to the community, though, so I don’t really see the need. This is because of perfectly sensible design choices made by both the Lemmy devs (using group: in webfinger to indicate groups, even though that URL scheme is nonstandard, so username and community can overlap) and the Mastodon devs (accounts follow the standard Webfinger notation and usernames are expected to be unique).
Mastodon has stupidities of its own (think “you must @mention usernames” despite ActivityPub having a non-content field for that purpose that’ll work just fine). But in this case the problem is that different projects use the same standard for different purposes.
Interaction between Mastodon and Lemmy is possible, but it’s a massive pain, and even if Mastodon were to support Page objects to render Lemmy posts, it’ll always remain a pain. I don’t think asking Mastodon to change the way their software works to support use cases it was never designed to support (and perhaps doesn’t want to support) is very viable.
That said, you could try to check out the code over at Github and see if you can make Page objects render better in Mastodon. Probably best to ask the maintainers what approach they’d prefer, but I think rendering posts would be a rather small change that would greatly improve interoperability.