A well-designed app that just works
I've tried many MQTT client apps but the GUI is usually counter-intuitive or just user-unfriendly. MQTT Loupe is a welcome exception to that and I had no trouble connecting to my own MQTT server, and receiving messages. It's also good to have logging to diagnose connections. I haven't used the certificates option yet but don't expect any issues. I particularly like the JSON presentation which again is clear and laid out with the user in mind.