In the real world, something is said to be mummified if it's preserved with most of it's skin and most of it's muscle tissue intact.
The concept of mummy rot stem from the fact that they still have tissue that potentially could have disease-carrying microorganisms preserved.
So I think you only should use the template on undead that have been buried for some time and have their flesh remaining. The difference from zombies is that they have been buried longer and aren't rotting at present in my opinion