Danielle gets a "sound lashing" from her stepmother just a day prior to the Mask. When she appears in her mother's dress, and the focus is on her appearance, from the back, there is no evidence of this lashing. No marks, nothing.
I just watched that scene and the back of the dress is almost to her neck, also her hair is long so you cannot see the whip marks. She does wince when he hugs her. If she didnt..then there would be a slip up.