A better name would be Feudiverse, since it's feudalism by design — complete with instances feuding over control of how the entire thing is run.
The admins of the instance have absolute control over accounts (i.e. people) that sign up with them. The instance admins are the law of the land, and can kick accounts off the net for whatever reason, as well as draw up the bridges and prohibit passing through their lands.
Like in any feudal system, people are judged by which Lord they belong to (i.e. which instance they sign up with), and will be treated accordingly. Each instance is a clan, with collective responsibility built-in.
There is no Federation without a Federal government that ensures rights of the citizens via a constitution.
And there's no Fediverse without a central account authority that limits what instance admins can do to any individual account.
Mastodon is a Feudiverse.