It is possible to be an admin of a Discourse site and not be a moderator. As admin, you may be very active on the site for reasons other than moderating. In this case you will not want to be notified about flags.
My suggestion would be to make whether a user is notified determined by a site setting, e.g. flag_notifications_allowed_group.
To keep current behavior, by default this setting would include admins and moderators. But it could be configured with any group that admins can join that want to be notified and help out with handling flags.