If you eat too many mashed potatoes then yes the mashed potatoes can cause weight gain especially if you add butter and other ingredients to the mashed potatoes.
Mashed potatoes are pretty high in calories so they're not an ideal food when trying to lose weight but you can still eat some mashed potatoes during weight loss but just avoid eating too many servings of mashed potatoes if you're trying to lose weight.
If you're trying to put on weight then eating mashed potatoes can help you gain weight if you eat enough of them.
If you make the mashed potatoes with milk and butter the mashed potatoes you make will contain 240 calories, 4 grams of protein, 9 grams of fat, 35 grams of carbohydrates, 3 grams of fiber, 12.5 milligrams of vitamin C and 600 milligrams of potassium.
Mashed potatoes are healthy though but if you're trying to lose weight then just don't overeat them.