"Getting dumber"? Dumber than what, exactly? Dumber than humanity was in the 19th century, when the vast majority of people educated their children in coal mines and factories, where they'd be spending the rest of their lives? Dumber than humanity was in the 20th century, where it was decades before academic education at the high-school grade was anything but a priviledge, and women and black people faced discriminatory policies that limited their progress through the academic system, policies that only began to change in the second half of the century?
Or dumber than ten, twenty, thirty years ago, when you were young, and people were good, and you grew up listening to your parents and grandparents talking about "how much better everything used to be", before this sudden wave of moral and mental decay that seems to have settled in amongst the unwashed masses. Back when blacks were inferior and homosexuality was a disease, and teenage rebellion was a glorious statement of the new era, and not just children being disrespectful. And there weren't these students and young adults who ran wild getting drunk simply to have a good time, because in the good old days people swarmed the pubs and bars at the end of the working week because it's the only way to forget that they had to go back to work in the morning, where they worked their dangerous jobs for unreasonable hours for a pitiful wage.
Because people who almost invariably have the ability to read, to write, to work with simple mathematics, who can use computers and video phones and devices that were unimaginable twenty years ago are just getting dumber. Because people who have the chance at an education, regardless of social or economic background, where they can learn about the science of the universe and what it means, because E=MC[sup]2[/sup] is common knowledge, and they can think about people and think about the world in terms of hard fact or mysterious spirituality because it's what they believe in, and not just because they'd be beaten if they didn't say their prayers, are just getting dumber.
No. People are people. This will not change over a few decades. This will not change over a few centuries. A species will not become any more or less intelligent over so few generations. What can change is society, and what we consider to be "basic knowledge"; people will move with the times, but humans will stay the same.
Correlating evidence: people are clearly still as arrogant as ever, if this thread is anything to go by.