Here's the thing. I'm a mod for a small-time community for a niche interest, [email protected] I'm also on Mastodon, and was before my Reddit exodus. I follow #castles as well as a few other related topics on Matsodon, so I get quality toots, such as this: https://mastodon.scot/@McNige/110926238926867959, that I wish I could just crosspost over to my community. Currently, I have to repackage the toot, which isn't a huge problem, but currently I just drop them a note on Mastodon that their content has been posted elsewhere on the Fediverse. What would be nice is if people who comment on the Lemmy post also get fed into OP's toot. More sharing, more connection, more activity.
On the flip side, I've subscribed to @[email protected] on my Mastodon instance and, while it's good to be able to follow posts in feed form, it looks like ass: 
I realize I should try this with Pixelfed, but I haven't made that leap yet.
I don't know, am I thinking crazy here? I'd think we'd want everything in the Fediverse soup interoperable in a more seemless way. Is this a feature request or am I missing some way to do this better?
Yes I do see communities boosting the comments too, they are marked as replies properly if I click on them. You're right though there's no way for Mastodon to follow a community without your feed getting filled with comments instead of just posts.
But I think the community needs to boost the comments in order for them to federate properly, so idk if there's a nice way that this could be solved.