In a drawing - or in a model? If it's a drawing, just create your own suffix and hide the unit string.
To be technically correct however, under the SI units definition, "milimeter" is abbreviated to "mm" not "MM". The first letter prefix "m" in lowercase means milli or 1/1000th of [unit of measure]. "M" however means "mega", or 1000,000 times [unit of measure]. So by writing "MM" to represent millimeters, you might end up with something rather larger than you expected. (Not sure if "M" is even a recognised unit of distance?)