You can search and find Mastodon accounts from here, bit it doesn’t seem to actually pull in posts and there’s no way to follow anything afaik. You can follow Lemmy communities (and post to them) from Mastodon, but the interface isn’t great and it’s kinda spammy.

Technically everything talks to everything else, it’s just a question of implementation of features and a decent UI for seeing content from other software that may be in different formats. Exciting to see where that aspect of the Fediverse ends up!