I don't think you should need to use losetup, however, if you are using Android 4.2, the path to your sdcard may be the issue. In Android 4.2, that has been changed due to support for multiusers on tablets. You would likely need the full path rather than the "shortcut" that has been set up for simplicity by Android. That path would be
But, I believe Android will support mounting it directly with the mount command (it is what I have used a few times in Linux, although, I've never tried in Android).
mount -o loop /storage/emulated/0/nandroid/...data.img /mnt/wherever
Keep in mind, you will need to be root and the directory you want to mount it to needs to exist before you run the command. And when you do mount it, if you mount it over a directory that has content in it, it will prevent access to that content (it isn't gone, but unaccessible until it is unmounted).