There have been some requests to make it possible for Mastodon users to create threads on Lemmy. At the moment this is not possible because Mastodon cant set the title field which is mandatory in Lemmy. The same problem affects other projects like Pleroma or Friendica (where title is optional).
It is unlikely that Mastodon would make any change in this regard, so a workaround in Lemmy would be necessary. I implemented such a workaround which makes the title optional when receiving a federated post, and uses the first line of the content instead. This workaround could also be made configurable per instance or per community.
But to be honest I am not sure the workaround is a good idea in general, as it could result in bad titles, and generally more low-quality posts. Thats why I want to know what the community thinks about it.
The Activitypub standard doesnt mandate any specific properties, not the title and not even the content which Mastodon sends. Thats one problem with Activitypub, you can send almost anything you want and its technically valid (even if no other platform can handle it). However the presence or lack of a title isnt really related to groups.