You can follow Lemmy communities on Mastodon but that will cause every single comment to appear in your feed, so depending on how much activity they get, they will probably flood your feed. You can post to Lemmy communities from Mastodon by mentioning them, to the best of my knowledge. In either case you can reply to whatever you see on Mastodon and it will appear as a Lemmy comment.

You cannot follow Mastodon accounts on Lemmy.

A gnome obviously

Do we have “circlejerk” or “shitposting” communities here on Lemmy already?

It was definitely a good development to let social media operators make automated “recommendations based on what you seem to like” instead of, you know, letting users decide manually what they want to see, amirite… /s