I think it's a good idea. I don't see any reason to limit which posts are available based on the presence/absence of simple fields. That's just a UI problem and using the first sentence (up to a certain character limit) as a title is a good solution. Savvy microblog users could even write their posts specifically for lemmy by writing a title and adding a newline, obviating the need for custom code in microblog software or lemmy
EDIT: Another reason in favor of this proposal just came to me. Mastodon has group support on their roadmap, but my worry is their team won't look at prior art and make their group support compatible with existing Group
implementations. If Lemmy allows external users to create post to its groups, mastodon users may start actively posting to lemmy groups. Then the users would expect any native mastodon group implementation to be compatible with what they're already using.