Not sure if anyone is still looking to change the default display on the mobile app, but I was able to by using the simulator in the IDE. I have three temp/humidity sensors that showed the temp by default. I chose a code template that required a humidity sensor, picked the sensor I wanted to change, then clicked on the config gear in the top right. I could then change which data tile to display, which also changed the default display in the mobile app.
Hope that helps!