Can mashed potatoes be made early and kept in crock pot to be kept warm?

Yes. Keeping mashed potatoes on low and covered in a crock pot is an Excellent way to keep them warm. Another idea (if the crock is busy with other things) is to use a hot water bath. Just place the bowl of mashed potatoes inside another bowl of warm to hot water. Use kettle water to replenish the heat of the water bath when necessary.