I’m not much help, as I’m having issues as well. I have the following in setup, but getting “offline” trying to check the status of my zigbee2mqtt addon:
You could just use the TCP port 1883 to see if the server is up. The MQTT monitor is only to monitor a specific topic for a certain message. Not what I think you want to accomplish.
From the “Monitor Type” select “TCP Port” and it should work.