MedlinePlus lists cow's milk, eggs, crustacean shellfish, fish, peanuts, sesame, soy, tree nuts, and wheat as the foods that most often cause food allergies. Mustard is not on that list.
Mustard seed and mustard-derived ingredients still appear in condiments, marinades, and processed meats. Some people do react.
Read full ingredient lists if mustard is a concern for you, and follow clinical advice for your situation.