I think I’ve figured it out. I used to read that same variable from a json packet read from a http page but it often timed out, hence why I wanted to try mqtt. I simply changed how it was configured in configuration.yaml.
I renamed the mqtt defined name from outside_temperature to acurite_outside_temperature and I do get a value now.
Well, nope, I didn’t figured it out. Now I think I did. In Weewx MQTT configuration, retain was false so the sensor was ‘unavailable’ until Weewx published its next set of data (which is every 5 minutes).