Thank you for your post. Of course, anyone can be a mystery shopper, at least with a tax ID number. Most of the people here have a goal of excellence in performing their task. After all, as you illustrate, someone's livelihood is being evaluated. The hardest part is understanding what the client wants for an evaluation and to set aside one's own personal preference. Took me a few times to get there.
Good luck on your quest.
Do not read so much, look about you and think of what you see there.
Richard Feynman-- letter to Ashok Arora, 4 January 1967, published in Perfectly Reasonable Deviations from the Beaten Track (2005) p. 230