DBSS - Database Mirroring Role requires user account with with sysadmin role.
1. We query IsMirroring metric base on sys.database_mirroring, to see non-NULL values on a mirror database, the Db user account must be a member of the sysadmin fixed server role.
Here are the explaination from microsoft.
https://docs.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-database-mirroring-transact-sql
2. In our deployment guide, we've informed that at the "Permissions for SQL Server databases" section, there is a note
"NOTE: Monitoring mirroring requires sysadmin privileges.".
