I will preface this with "I am not a taxidermist", however I have skinned a few birds and have watched some videos and a few taxidermists prep skins.
You need to get all the meat and as much fat off the bird as possible. You get the meat off for the reason that it is flesh. It will rot and it attracts bugs. The reason you get the fat off is it is oily. When waterfowl are not defatted, the fat left on the skin will eventually start to melt. When it does, it will permeate the skin and show on the feathers. When this happens, you effectively have an oil slick on the bottom of your bird that is rather unattactive and destroys the mount. The only way to win the war with that is to remove the source...but one the bird is mounted and the skin is dry, there really isn't much you can do.
Hopefully one of the real taxidermists will chime in here for ya.
There are no ducks in the desert!