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.
No a user’s page won’t be a community, it’ll be a self-post / self-comment, and you’ll be able to follow not just communities, but users. Mastodon doesn’t support community-following just as we don’t support user-following (yet), so that wouldn’t help.
My point is that automatically making a community by the name of the username on sign up would work like following users except itd allow someone to post on someones community which in turn would be like send a toot to a specific user on mastodon. Following a users “community” on lemmy would be following the user on mastodon.
You could do that at any time I spose, but the main difference is that usually communities are topic focused rather than person-focused, which is one of the main differences between the reddit and twitter models.