The large "Kroger" chain in my neighborhood has changed their open hours from 24/7 to 8A-8P. I expect many other stores will be doing the same as I am sure many of the employees will be either choosing not to or will not be able to come in soon. That will definitely contribute to the havoc out there. And yes, under these circumstances the report will not only be not very useful to the client, but might endanger the mystery shopper by asking them to be part of the crowd.
If things get really bad I envision many stores will go to delivery. Several retail stores and restaurants in my area are waiving their delivery fees and I imagine groceries might try to switch to this as well if the number of cases goes up.
So just make sure to check your assignment page before going out on any mystery shops. Make sure the shop is still there.