Currently the MairadMon documentation says that the REPLICATION CLIENT privilege is needed as a minimum:
GRANT REPLICATION CLIENT on . to 'monitor_user'@'%';
Assuming that this privilege is mostly needed to execute SHOW SLAVE STATUS it needs to be replaced with
GRANT REPLICATION SLAVE ADMIN ON . to 'monitor_user'@'%'
for server versions 10.5 and later.