I highly doubt that. Media ia a business. Business cares only for profits. Of course, if people are dumber they are more likely to watch their horrible programming but I don't see a conspiracy there. Reality TV is cheaper to produce and creates more profit. Bad television in general is cheaper to produce because you don't have to hire very talented people to make it. How does watching a stupid show on TV dumb you down exactly? I'll watch something stupid occasionally just to laugh at how stupid it is.
I believe media is a reflection of a society, not a major influencer. People are pretty dumb on average so the most popular shows are not of high quality or substance. They are also living extremely hectic lifestyles that keep them away from the tv so content changes to reflect that. You don't have to watch every episode of the Kardashians or Dancing With Losers whatever to "enjoy" an episode so there's less episodic shows that require more of an investment from viewers.
I definitely agree there is good content on TV. Not a lot, but for someone to claim it's all worthless is snobbish and just not true. Bottom line is that the corporations who control our media are going to create whatever shows get ratings. If the majority of tv viewers were intelligent and only watched high-quality informative shows then the content would be created to capture that audience. Some networks such as AMC and HBO cash in by catering to this niche market. Most of TV is crap because people are too busy to care, distracted by the internet, and just not very smart.