Yes it is and it's a reasonably easy thing to measure on a large scale.
lol, easiness to measure has nothing to do with accuracy of individual results.
If it was truly a measure of intelligence, it wouldn't change based on educational levels. But it does. IQ tests are just a measure of how much schooling you've had.
Now, building on that point, if most of the bible belt states have lower IQ scores, you can NOT assume a causal link without further study. There is no evidence suggesting that because their IQ scores are lower they are more religious. But since IQ tests change based on how much education you've had, you may hypothesize that perhaps the lower standards of education in those states can lead to ignorance about science as well as lower IQ scores. Of course this is only a hypothesis, but it relies on far fewer assumptions than what you were trying to say.
Point is, religiosity does not affect intelligence directly. Stop being a prick.