You can't simply drop data into a mashup. First, you would add a proper widget to the mashup. The widgets task is to render the data. Second you need to create a binding between the data and a property of the widget. For this, select something from the Returned Data section of the service call, and drop it onto either the widget or directly a widget property on the left side. Only the properties with ingoing bind direction and a matching data type will accept the binding.
Check this tutorial for details: https://community.ptc.com/t5/IoT-Tips/Create-Your-Application-UI-Guide-Part-1/ta-p/809008
