I do mystery shops occasionally. As others have mentioned, the pay is often low for the time. Also, remember you would be a contract worker responsible for your own taxes. Rarely, did I make enough to make a difference in my taxes. People forget to consider expenses such as gas and wear on your vehicle. If I schedule a shop, I try to be sure it's close or on the route where I am going that day. Still, it's nice when the payments come in.
ETA: I still do oil change shops. I've done so many, they're easy for me and I would pay for the oil change anyway.