If I'm not mistaken even though you can view users within the fediverse you won't get anything because the instance of Mastodon does not store data the same way that Lemmy does.
Think of it as someone went into a cheese factory asking to talk to a specific person who works there and asking them to see their selection of beans
So, as I understand it based on other systems I've used/worked on that had a few similar feature sets, ActivityPub will effectively send out events of "things". What that is depends on whoever is telling ActivityPub about it. So, the data for a mastadon post (what are these called?) Would likely be pretty different to the data shape of a post on Lemmy. Not specifically the same, but I don't think there's anything stopping somebody from developing a server that can handle multiple types of content. Kbin has a similar thing between threads and microblogging. Two content types supported there.