Construct doesn't have the option to specify ldpi, mdpi, hdp or xdpi assets as native code does, but when exporting to cocoons you must select full screen mode, so the app will stretch to cover the whole screen on the device..
You can design in the biggest res (will consume a lot of ram and may crash the device - happened to me) and it will scale down with good detail
You can design low res and it will scale up, but get blurry.