Mastadon is just an instance on the Fediverse. It's an instance run by a power-tripping little bitch, but it's just one instance.

It unfortunately has good branding and has gotten people (mostly normies) to call most of Fedi "Mastodon", which is incorrect. That'd be like saying the site with the little blue bird that Elon bought last year is called Nitter because someone posted a Nitter link to a post there.