The request for a "Read Only" API viewer makes sense.
However, I'm not sure if this should be part of the "Backup Viewer" role, as the Backup Viewer is intended to only see backups.
It might be possible to add such a role when our new v13 feature "Custom Role (RBAC)" adds support for REST API.
I'll discuss this request further with our REST API team.
As a workaround, you may check out RestAPI of Service Provider Console. VSPC already allows Read Only API access.