Marilyn Monroe was one of the most successful actresses of her time. She knew what men found attractive and what women admired and that is what she became. Marilyn broke barriers for women and for race. She was not the ditsy blond she portrayed for she would not have been as successful in the world if she was.
Yes, Marilyn Monroe was smarter than people thought. "Dumb blonde" was just part of the image she portrayed in order to find her own position in the entertainment industry. She, and likely advisers, knew that a sexy blonde would be appreciated more by her target audience of white men if she were to appear less intelligent.
Yes, I think that Marilyn Monroe was smarter than a lot of people thought. Unfortunately, she decided to take on a bimbo persona to increase her popularity. It's a shame, because I wish she would have known that it's possible to be successful even if you are both beautiful and smart.
The assumption that Marilyn Monroe was a "dumb blonde" is probably more a result of sexism than anything else. She played the role of a dumb blonde because that's what people wanted and what sold at the time. She exploited that fact to make money. I wasn't alive at the time, but I assume that she was a savvy businesswoman and definitely a lot smarter than people assumed.