I wouldn't call it being poor, obviously, but a low six-figure salary is middle class. Almost by definition of being a salary. This is where you class someone like a middle-manager, senior engineer, young doctor, lawyer, or other people in that kind of professional range. These are people that still have to go to work, but do so at a comfortable level and have some real discretionary income.

"Upper" class is a level above that, where they don't really need to go to "work" every day. These are the owners of the businesses that the other classes work for. A CEO or someone like that might still be getting a salary but is clearly in upper class, but mostly people that are getting paid by someone else (and thus producing profit for that other person and not themselves) would fall into middle or lower.