ActivityPub is so loosely designed (in my opinion as somewhat of an outsider) that the opportunity was squandered to be able to have all different services interact smoothly with each other. It’s basically one little fiefdom per app, and if Pixelfed wants to make itself compatible with Mastodon’s fiefdom, then fine, and likewise for Mbin with Lemmy and etc, but it’s not really “cross compatible” between the whole universe of apps, in the same way as other better-designed protocols like email work, where it’s just “email” with no app specificity to it. It is a shame and a missed opportunity with how the protocol was designed, I think.

I think in general, the fediverse people are working on solutions, but we’re sort of stuck into the present setup which has this not really ideal compartmentalization and there’s not a good way to fix it. Certainly not from the Lemmy side that I’m aware of. Two possibilities though:

• Pixelfed has in-progress work on groups support, which is apparently very close to reality and at which point it should be able to interoperate with both Mastodon and Lemmy, which are the two main fiefdoms at present. And, Dan seems like he takes it pretty seriously the idea of maintaining compatibility all around (incl specifically with Lemmy).
• You can do kbin/mbin, which is more “Lemmy like” and is the best currently-working option I’m aware of for interoperating with Mastodon and Lemmy both (incl following Mrs. Hedge on Pixelfed and also any Lemmy communities). That’s why I am using mbin currently FWIW.