Yes, Walmart does monitor its security cameras but not constantly. In areas of high crime, security footage is more likely to be consistently monitored but for most states, Walmart uses its security cameras as a tool to support potential accusations and police investigations, rather than for instant theft prevention.
The only locations in a Walmart that doesn't have security cameras are: the restrooms, the dressing rooms for trying on clothes, and the employee break room. Every other location is watched by at least one, most often multiple cameras. Every register has it's own camera.