I don't see why it's wrong. You might want to do it in one-minute intervals, stirring between, to make sure they're evenly warming and not drying out. You could throw in a bit of extra stock or butter to prevent that as well.
An oven is another option, if that's possible. Here's a tip about reheating mashed potatoes from a post of ours:
A crock pot can serve as an excellent warming vehicle before your dinner. For reheating the next day, simply throw into a casserole dish, cover with foil, and reheat in a low oven until warm.
Just an fyi..it's advised that you not use a slow cooker to reheat items. They are in the danger zone for too long.
http://www.extension.umn.edu/food/food-safety/preserving/safe-meals/slow-cooker-safety/
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An oven is another option, if that's possible. Here's a tip about reheating mashed potatoes from a post of ours:
A crock pot can serve as an excellent warming vehicle before your dinner. For reheating the next day, simply throw into a casserole dish, cover with foil, and reheat in a low oven until warm.
https://food52.com/blog/9048-mashed-potato-strategies
http://www.extension.umn.edu/food/food-safety/preserving/safe-meals/slow-cooker-safety/