Ha ha, um, well... I've been watching movies at a rate of roughly one per night for the last few years...
And I would have to say no. I've seen lots of very intelligent films, and a staggering number of utterly idiotic ones. This is, of course, not counting the bottom 10%.
Not to say that American cinema is dumb, it's just that it has hosted a very large number of very poor movies that have tipped the average. There are some very talented and creative filmmakers, it's just that they take longer to actually make something, taking up time that the less talented spend using the same slightly moldy script over and over again.
However, all this pales in comparison with the juggernaut of bad taste that is Indian cinema. The region around Andhra Pradesh alone makes roughly three movies a day, and they all share the same "plot", acted out by people that not even a soap opera would hire.
In India, a film's success is determined by how impressive of a song they manage to pull together for it. Regardless of the film's setting, plot, actors or funding, every Indian movie will have time for at least one scene where everybody starts singing and dancing. And they always dub movies. Not into a different language, they just dub it. From Telugu to Telugu, from Hindi to Hindi. Why they do it is beyond my comprehension.
*Ahem*... So, anyways... No, I would have to say that the middle 80% of American cinema is not intellectually stimulating. However, I also think that that is a very large percentage, and that it does cover some intellectually stimulating films. It's not just the upper 10%.