My understanding of the current state is that with your Lemmy account, you can only interact with Lemmy-compatible activitypub services (Lemmy, Mbin, PieFed, Sublinks). Other services may be able to interact with Lemmy (I believe Mastodon and Friendica can, and Misskey may be able to as well).
There is no reason that Lemmy couldn’t interact with other content, but it gets a little confusing when you are talking about following users and not communities, Lemmy doesn’t work that way. You can’t even follow another Lemmy user. So no one has implemented this in Lemmy yet.
Long store short, no you can’t access in that direction yet, but there’s no real reason why that couldn’t come in the future. You could check out Mbin, which I believe supports more services. You can interact with Lemmy from other services (such as from Mastodon), but it gets a bit weird sometimes as they are different kinds of services.
I’m interacting with my Mastodon account using my lemmy instance and vice versa…
I’m getting back to you from my Mastodon instance… Wait !!
Yes my understanding is that you can use Mastodon to post to a community (by tagging it) or to respond to a post or comment. You can follow a Lemmy community as if it were a user.
However, to use a Lemmy account to interact with Mastodon, you are limited to replying to Mastodon users who have posted in a Lemmy community or commented on a Lemmy post.
