Marietta was always a bit anal about keeping herself clean. She was known for being overly concerned about a little dirt and grime. She couldn't stand it. She'd wash her sheets on a daily basis if Padrig would let her get away with it.
The idea of a stain forced her to take the food with her into the muggle restroom, setting it down on one sink and she scrambled to scrub at the next. She wasn't going to leave until she was sure it wouldn't be there.
Umbridge would just have to believe it took that long to get the food.