How is mutable/immutable data handled on AtomicHub?

Assets can already have mutable data (attributes), but it's only supported on contract level, not yet within the NFT Creator (right now all attributes are auto-created as immutable).

Authorized accounts can change mutable data at any time.

If a certain attribute is already defined in a template's data, mutable data still won’t change it.

You can update the assets mutable data regardless of whether it uses a template or not. You just can't overwrite attributes that are already set by the template (precedence: template data > immutable asset data > mutable asset data).