The developers behind Lemmy are doing great work on this front of adapting ActivityPub to be usable by community software:
However, they had to extend it, so there is no other software implementing their activities.
Our federation implementation is already feature complete, but so far we haven’t focused at all on complying with the ActivityPub spec. As such, Lemmy is likely not compatible with implementations which expect to send and receive valid activities.
I guess if Lemmy can get it to be compatible with Mastodon, we can go and implement the same API Lemmy uses, mapping