What is incumbent upon one who removed her deceased mother's clothes to clean them, thinking that this act was correct, and then someone alerted her to her mistake?

It is forbidden to pluck the hair of the deceased with force if that would cause pain were a living person to do it; because what harms the living harms the dead.

Therefore, the daughter who did this to her mother must repent to Allah, and increase in righteous deeds, supplication, and seeking forgiveness for her mother.