"The TV is dumbed down." When I read "the TV", I think of the device and not the programming. You could say "Modern television is dumbed down" to be clearer. "Modern society has dumbed people down." This is kind-of OK, but I want to know what part of modern society has done this? All of it? In what way? This is straight from the internet: "Has social networking and texting dumbed people down in general?" More specific examples make a better sentence, even through technically your sentence is fine.

The expression is not useful. Most of programs shown on television are aimed at those who consume the products sold by the advertisers. Sitcoms sell breakfast cereal and older versions of the automobiles advertised.