@jwz It's a distinct failing of the system, I think. In theory, you can follow mastodon accounts on pixelfed, and vice versa, but the affordances live with the base service/UI that you sign up for. There's no federated identity where (e.g.) a mastodon account can be used to give you combined access to a pixelfed (or other) service.
It's one of the reasons I haven't signed up for any other Fedi services -- I can't be arsed to manage multiple logins.