No, it hasn't made us dumber. Just as industrial agriculture hasn't made us "dumber" because most of us don't know how to kill a bison with a single spear thrust or when to plant yellow squash to maximize crop yields.
There's a difference between intelligence and skill. Spear-throwing and literacy are skills, and while spear-throwing was an essential skill for tens of thousands of years, widespread literacy has only been around for a couple hundred. Both take a certain kind of intellectual processing to do, but the lack of either skill, by itself, is no indication that you are overall less intelligent than someone who does possess those skills. You've heard it said that the victors write history to suit themselves? Well, writers get to write cultural history in such a way that "intelligence" has become equivalent to "able to quickly and accurately manipulate symbol systems". But as that speed and accuracy has been outsourced to computers, and as local and global economies become increasingly interdependent, for many people, mere symbol manipulation isn't nearly as important a skill as it used to be. Hence the emergence of ideas like Gardner's multiple categories of intelligences.
Eventually, continuous advances in computers and perceptual technology will probably reduce literacy to a quaint skill/hobby like calligraphy, or spinning flax into thread, or COBOL.