With the Epidemic, Undead Pokemon should be in 'Advance Decay' (but of course, there are several factors that can affect this such as; Access to scavengers, humidity, temperature, oxygen, body size, etc. etc.)
Also, another thing that bothered me is smell. If one were to cover themselves in Undead muck, would they be mistaken as Undead in an Undead's eyes? Or would they be easily seen as a living? -Just a reference to the Walking Dead-
Thank you for reading.