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.
This mistress could not go back into her car with her dirty feet on. She had to do something to make sure that she she had them cleaned before she did. So she waited for a slave or loser to pass by as she smoked. She found one and she enjoyed making him lick her feet and her soles clean. When she was as clean as she wanted, she went home.