OK Jordan, I played around with the AppStudio - Quick Report template...created two different applications:
- First one used ESRI cloud for storing the point FeatureClass...which I created from scratch including lots of fields.
- The Second one used my internal Server URL for the internally-hosted FeatureClass...different from the first, but still including a lot of extra fields.
In both cases, once I published the App, the fields in the FeatureServices cannot be modified. I suspect that re-publishing the service with some fields turned on, others/most turned off, is the best way to accomplish the appearance of 'turning off fields' in the Quick Report template.
Remember, the features and the AppStudio functionality is evolving, so this answer will not always be the most current solution.
Hopes this helps until then,
Jason