I don't quite know where flying instructors are on the scale of things, but mine (in a busy school) seemed to do quite well - certainly better than the average supermarket employee.
I'm only a part timer(4 days per week), but my net income last tax year was -£400. Yes, that is a minus. I doubt many supermarket employees pay to go to work
Perhaps at a very busy school in a good weather year an instructor might be able to scrape out a meagre living, but they would almost certainly do better at ASDA.