First, the citizenship test does not test intellect. It tests mostly knowledge of history, English, and US government.
Second, I've heard a lot of criticism surrounding the fairness of the test given that many natural born citizens can't even pass it as well as what language it should be given in.
Third, you're talking how many thousands vs how many tens of millions?
And what's going to constitute a "non-idiot"?
If a business issue is on the table for an election, how much knowledge do I have to have about the mortgage crisis?
How we fight Afghanistan is extremely important. So does somebody have to understand military logistics not to be considered an idiot?
I think you see where I'm going and I just don't think it's reasonably practical to administer such a test.