No, you just have to be paid, so that way you're a "professional SO"!
The reason so many SO's are MM (and SS) is because there a lot of them who are invested in the continuation of the sport. Many of the EX, MA & DM shooters care more about their own personal performance than what it takes to put on a quality event. A small percentage of them may work one sanctioned match a year. At that rate, most who shoot major matches will never see them behind a timer. The funny thing is when you hear all the shooters who complain that SO's have it easy as shooters. If that were true, we'd be knee deep in MA & DM SO's!
FWIW, it's a LOT easier to complain about the quality of the officiating, than it is to do something about it. Do the complainers ever offer to work as an SOI? Do they offer to mentor SO's at matches? Do they offer to hold SO/Shooter clinics so the SO's get a different perspective on running shooters? Do they invite SO's to practice days? The answer is almost always no. So remember when someone is complaining about your lot in life, it most likely means they're not happy with THEIR lot in life. You walk away smiling, they don't. Which would you rather be?