Don't believe that's how it works. It's not SSO (which would be AWESOME). That will create an actual local account on other servers. This is still your mastodon account 'viewing' a pixelfed account via the ActivityPub protocol. It's just the 'view' that's different for different services (very broad definition). Viewing a pixelfed account/profile via a pixelfed instance shows you a photo gallery that has text with each photo (not alt text). Viewing that same pixelfed account from a mastodon account, you'd see a series of toots/posts text that happen to mostly have pictures. The reverse view of pixelfed looking at a Mastodon account, not sure but I'd guess just a mostly empty photo gallery with text associated?