@ShittyKopper rightly mastodon cannot and should not aim for full compatibility with Lemmy and Piefed.
In fact, Lemmy has some unique features (such as the intentionally limited management of crossposting) that cannot be managed.
However, it would be appropriate and above all ethical, for Mastodon groups to be compatible with Friendica groups, Gup groups and Mbin magazines.
This would allow mastodon users to continue using the Activity pub groups of other platforms, including Lemmy.
I remember in fact that even WordPress (which at the moment, represents the most important novelty of the Fediverse) is perfectly able to publish on the Lemmy communities and on the Friendica groups!