Luckily stupid isn't genetic, so I don't think devolution, at least in a literal sense, is happening. As for whether people are getting stupider, it's hard to say. On one hand, with the progression of society we naturally get a better scientific understanding of the world around us. On the other hand, the media makes a majority of its business pandering to and encouraging stupidity. (See Jersey Shore, Justin Beiber, Pretty much anything that's been on MTV for the last 6 years, The Twilight Movies, Paris Hilton)
It creates this weird double standard wherein we encourage progression as a society yet put stupidity up on a pedestal. I honestly don't know what the result will be. I would say it might have some basis, but not enough to make it seem like an inevitability, since stupid people haven't shown statistically to raise stupid kids.