Your Mastodon login is tied to the Mastodon server that you belong to. Mastodon servers are run by volunteers and servers can disappear.
What happens to your Pixelfed account if your Mastodon server shuts down?
What happens to your Pixelfed account if you switch to a different Mastodon server?
What happens to your ability to log into your Pixelfed account should your Mastodon server be down for maintenance?
What happens to your Pixelfed account should you be suspended by the Admin on your Mastodon server?
A Federated Fediverse login would need to be tied to a central entity that is permanent and always available. No such entity exists and will probably ever exist. I'd be cautious using a Mastodon login on any other platform unless someone can convince me I could recover my account should my login-affiliated Mastodon server become unavailable.