There is no standard for mule ducks. They are sometimes raised commercially but as they are sterile, someone would need to keep purebred flocks and then mix the breeds to create hybrids.
Mules are muscovy crossed with any, usually large breed, mallard derived breed. If you cross a Muscovy male with a Pekin female you will hatch moulards (or mule ducks); if you cross a Pekin male with a Muscovy female the progeny are called hinnies. Hinnies will have differing sizes in the genders, moulards will have similar sized male/female. It is said that these crosses have very low fertility rates and in Europe, they use artificial insemination to increase fertility %.
You can raise any large breed duck for meat. Pekin, properly raised, are not overly fatty. But they will take a little longer to grow to size with exercise and a limited diet.