Potatoes are healthy for you and eating mashed potatoes can be healthy too.
But when you add too much butter or other flavoring to the mashed potatoes then you add on the calories which makes the mashed potatoes less healthy.
Still mashed potatoes do provide you with nutrients needed to keep healthy but avoid adding too much butter or other stuff to it to flavor it to keep the mashed potatoes healthy.
I eat mashed potatoes a lot myself and I've been healthy but I avoid too much butter or other things in the mashed potatoes.