What is a Munchkin dog mix of?

There is no such thing as a "Munchkin dog mix".

The term "Munchkin" refers to a breed of cat, not a dog. Munchkin cats are known for their short legs due to a genetic mutation.

It's possible you are thinking of a different breed or mix, or you may be confusing the terms. If you can provide more information about the dog you're interested in, I might be able to help identify it.