'Little Boxes' doesn't criticize middle class success per se, but rather the scourge of the U.S. suburban housing development, and through that the social irresponsibility of any successful middle class people who chose to use their wealth to support such a blight.

The problem of the suburban housing development is not the bland sameness of the houses, but the degradation of both the city and the countryside, and the alienation of city dwellers from the natural world as the borders of cities are increased exponentially, plus the extreme demands it places on fossil fuels due to the long distances that suburb dwellers have to travel and the impossibility of serving such a widely spread population with public transport (not to mention the long distances that utilities and services have to travel to supply the suburban house), as well as things like the loss of valuable farmland and the transformation from a walking, interactive lifestyle to one of car-driving, social isolation, and lack of exercise, and let's not forget the racism and general misanthropy and egotism that motivated and are encouraged by the move to the suburbs, and the militant imperialism needed to supply such a culture with the resources it so wastefully consumes.

But it's difficult to express all that in a song, so people tend to focus on the sham aristocracy in the visual appearance, as Kurt Vonnegut did also, describing a development of "three thousand dream houses for three thousand families with presumably identical dreams."