It is wrong to reheat mashed potatoes in the microwave
It is not! Go right ahead. If you wanted to do it in a low oven, that works too.
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.
I've done it before and seemed okay, but a lot of chefs say it's a bad idea. I'm going for it. Thanks!!!!
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.