In the WebGUI > Apps > Installed Applications, try clicking the 3 dots next to 'Mosquitto' and choose 'Shell'. Here's a similar menu for my Grafana App:
From the App's shell, you should be able to find 'mosquitto.conf' wherever the App developer "normally" puts it e.g. /etc/mosquitto.conf
You might also consider editing your Mosquitto App's configuration to WebGUI > Storage and Persistence > Type of Storage = Host Path (simple) , and then map the App's /etc directory to some directory on your TrueNAS SCALE host's filesystem, if you would like to be able to access the App's /etc directory outside of the App's shell. This would also mean that the App's entire /etc directory would persist if the App was deleted.