I do not think you can set a number format by default to use for measures in the front end. You could however create master measures and use the Num() function to set a default number format for that specific measure. Then you can drag and drop these master measures in the visualizations.
Also want to avoid this. Have the opinion a master-item should not be formated. The format should be taken in each object individually. It looks like Qlik Sense has not such option. Would like to shift this thread in the ideas-forum. Do I have to post a new thread there? Didn't find out how to move it.