The reason that mashed potatoes become wallpaper paste when you put them in the mixer, or use a hand mixer or blender is because of starch. Starch is your enemy when it comes to making creamy mashed potatoes. So how do we fix this mess and still end up with smooth and creamy potatoes? Add a little longer to your prep time and a little less to your mashing time.
When peeling and boiling your potatoes, take the extra step and cut your potatoes in thin, chip like slices. You slice your potatoes as thin as you would like, but try to be consistent in the thickness of your potatoes because the thinner they are, the faster they will cook and if you have a wide variety of thicknesses you will end up with over done potatoes and potatoes that are still hard in the center. I like to do mine about three times as thick as a potato chip.
Once you have your potatoes slices, let them soak for about an hour in a pot of cold water. If you think about it during the time they are soaking, change the water a couple times. Helpful, but not necessary. Then right before you start to boil your potatoes, change the water one last time.
The reason for doing this is because slicing them thin will expose as much surface area of the potato as possible for the upcoming soak, allowing more starch to escape the potatoes, and the water that you are changing will have a milky white tint. That tint is the starch escaping from the potatoes and being washed down the drain.
Once your potatoes are cooked through, do the extra work and use a potato masher. Since you potatoes are sliced so thinly, they will be easy to mash and will leave you with fewer, and smaller lumps. If you must use a mixer, use it for as short of a time as possible, but the thin slicing and subsequent soak will have removed a good bit of the starch from the potatoes, which will make them less likely to have that paste like texture. Then simply add the things that you would normally add to your potatoes and enjoy. I like to add milk, butter, salt, pepper and sour cream. All of which to taste.