If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
So people don’t have to maintain accounts on random services. Personally, I’d quite like to see new posts from Lemmy communities in my Mastodon feed.
These different services exist for different reasons and are specialized and different from each other. Mastodon interface is good for exactly one thing: microblogging. Lemmy is for link aggregation and discussion, PeerTube is for videos, etc etc. Tell me, how do you give a PeerTube video a thumbs-down on Mastodon?
Alright, I’ll bite: that’s just how I use the internet, you’re free to do whatever you want. Regarding your question, I’ve never seen a need to do that, so I’m not sure. Have you tried this yourself?
Of course you’re allowed to do this. I’m commenting mostly on the insistence that federation with Mastodon be a priority for devs.