Personally, I think it was both Connie's naive nature and vanity and a little bit of societal expectations that made Connie an easy target. Connie is a typical teenager for this time period in America. She is into boys and music and her looks, but she is also very vain, deceitful, and manipulative because she is attractive. Her vanity helps set her up for Arnold Friend because he is able to get her attention by flattering her. She is not distrusting at first when she should have been alarmed from the beginning! I believe her vanity and inability to look past the flattery is what "did" Connie in, not societal expectations and/or roles.
Young women were expected to be proper, studious, and to have manners. Women were primarily expected to be at home taking care of children and the home. Because of this, perhaps many women were naive because they were not out in society that much and were not exposed to much of what was bad; therefore, they were more trusting.
Connie is sexually inexperience, but she want to explore to make that part not true and Arnold looks like the boys she wants. While Arnold finds her pretty he uses that to his advantage.