Where I come from people call them whipped potatoes, the smooth milky ones that is. My mom used to make them that way--milk, butter, whipped with the electric mixer, etc. I prefer smashed potatoes with big lumps, fresh cracked black pepper, sea salt and a drizzle of extra virgin olive oil.
By the way, if you like mashed sweet potato, bake the potatoes instead. Roll them out of their skins, mash with a fork, and if you're really feeling like a treat, mash them with some roasted butternut squash and stir in a little garlic butter. Very nice!