Stadia will dynamically adjust the stream down to 720p when it senses that is the best your connection can handle, but it will not go lower than that.
You can force this to occur via the Stadia app:
By default it is set to "Best visual quality", which will automatically stream the best it can up to 4K, dependent on device and connection. You can, however, select "Limited data usage" to restrict the stream to 720p.