Marilynn Monroe was an actress. The days and times in which she lived and worked were very incongruous with the image that she portrayed. In the 1950s and 1960s, social rules were stricter and more rigid than those of today.
The thing that made her a cultural icon was not so much the roles she played on the movie screen but the role that she played as a person. In a culture where most people were extremely private about issues such as sexuality and identity, she gave the impression that she was comfortable with who she was regardless of what anyone else thought about her. While that may not seem unusual to a person in 2015 America, it was revolutionary then.
She was taken advantage of by many, she was misunderstood by most and she ended her life in a very tragic way because of the fact that she did not "fit" into the culture's mold. But even in her short life she broke and helped to reshape the mold itself.