"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.