Walking in mud and dirt is not something Madame Marissa is afraid of. She does it because she knows she is not the one who will clean her feet. She knows what when she reaches home, her slave will lick her feet clean before cleaning it and finally running her bath. Those are the things which make her feel great and powerful because it means she has power over someone.
Vivien had old shoes which she wanted to throw away. But she did not throw them away because they were smelly and she likes to use them to humiliate her slaves. She enjoys making slaves lick those old dirty shoes the way he wants. She always gets what she wants when she uses that particular punishment method. She wanted to use it today and she went looking for the shoes.