Read-Only permissions is not enough for the monitoring.
Specific collections used by Always On monitoring, in addition to read permission requires VIEW ANY DEFINITION, VIEW SERVER STATE permissions on the server instance.
You can grant these permissions in the trusted user script on SC documentation page here
The Trusted User (User which gets permissions from ‘trusted user script’ above) can see data in the Availability Group tab and in Secondary replicas as well.
In general we advise to use system administrator permissions.
If the Spotlight Cloud user that you use for monitoring doesn't have the minimum permissions as shown in the script on the page above, please grant the permissions, as they are required.