It's important to note that the New Testament is very different in style from the Quran. The gospels are written as accounts of history (More or less), with the core aspects of Christian(most denominations) faith (theology) being largely non-scriptural.[I could get in to this if you want me to] The Quran on the other hand is really just pure theology with bits of historical claims thrown in. It's written by God in the first person telling you what to do, how great and right he is, And the future consequences of your rightness or wrongness. (There is more to it than this, but it's not particularly important for you're question.)

Those differences definitely lead to a difference in the mentality of adherents. Speaking generally, Christians are smarter (or seem smarter) because the teachings of Jesus are mostly focused on morality and don't preclude people at all from studying science and whatnot. Aspects of theology like the trinity, creationism, or salvation through Jesus, regardless of how scriptural they are or how much one believes in them, are irrelevant because they don't have an influence on how one acts (except when they have to argue with atheists about them). I guess what I'm saying is that out of all Christians very few of them are actually as dumb as you think they are, and the ones that are stupid are only stupid because they're bad and they’re only bad because of bad theology which creationism isn't.

Muslims on the other hand could be called stupid because it would be much smarter for them just to believe in Jesus, and the reason they don't is that Muhammed was dangerously mistaken. [Sorry I wrote all this pretty quickly, please ask for clarification]