Yes you can earn money being a mystery shopper if you look around and do research on the mystery shopper companies before signing up.
My sister made around $200.00 extra per month doing some mystery shopping part time a few years ago.
I can't remember the name of the mystery shopper company she worked for but the company was legit and she did actually get paid for the mystery shopping she did so there are some legit mystery shopping companies out there if you do your research and look up reviews on the company.