Evolution is not a thing that favors improvement, but rather adaptation. People getting dumber is therefore not some form of "devolution" (for lack of actual words), it is just as much evolution as plants that gain protection against predators or animals that gain the ability to regrow teeth infinetely. Therefore, people getting dumber is not necessarily a reason for them to die out (unfortunately).
To get back to the actual topic; I definitely think people are getting dumber and dumber. I attribute this to a few causes;
-First of all our society. The most popular form is that of capitalism, where supply and demand determine the market. Capitalism favors the biggest group, because that is where the money is. Sure, they may not all have much of it, but if you can sell your products to two people instead of one (disregarding their financial state) that is the way to go. On the other hand, you also want to make a product that is easy to create and doesnt require countless hours of research just to get it to sell. In other words, you want people who are not picky and are big in number. Hence, your solution is to go for people who are not exactly all that intelligent. If you cover everything that is around average and below, you can get yourself easily up to 60-80% of all people as a potential target audience.
Unfortunately, this means that people of higher intelligence are ignored quite often, since they are as a group simply not that interesting. Hence, "entry media" (as I like to call it) are all that is being created, while the more complex outings are either ignored or not made at all. Games, I feel, are a prime example of this.
-Then we have our own nature. The problem is that everyone is still primitive at heart; we compete with each other and block out things that are alien to our environment. Quite curuiously, people with high intellect are actually considered aliens as well. Reasons can be; they work more effectively, they are faster, they ask a lot of questions. These are all things that have a good chance of incurring the anger (maybe even wrath) of your colleagues who feel threatened. In some occassions, you might even get the manager on your case as well, for the abovementioned reasons. Apparantly, in a Dutch study done last year, about 20% of people with high intellect cant find a good place to function thanks to the reasons above. Apparantly, a lot more of them are not able to employ themselves fully, most probably due to them adapting to their environment and "keeping quiet". I was not able to find a figure on that though.
In other words, our current society seems to shun people of higher intellect. They are not important as a target audience and (can) give threatening impressions on other areas. With everything working in favour of the people of average intellect (or lower), it is no wonder society seems to get dumber and dumber.