Not only is it inaccurate; it seems to be the opposite of what is really the case. That is, blondes might actually be quite a bit smarter than average - at least when it comes to money! A University of Queensland study of 13000 women revealed that blondes tend to earn 7% more than other women and also generally marry richer men.
An academic who ran the study thinks these results might be due to the strong association between blondeness and beauty (as opposed to blondeness and lower intelligence). But you never know, the latter (reputed) quality might have something to do with the results - at least as it pertains to marriage. Maybe those rich men want a beautiful and intellectually non-threatening wife?