Cindy's a well-rounded as a character so far. She has significant flaws, but at the same time, she's cheerful, initiative, curious, and emotionally intelligent. She can read Ian when something is bothering him, support him in his struggles, and even defend him from the other friends in the group when they rip on him for his lack of professional success. I don't think that's what makes her that popular, though.
Being modeled after Nancy Ace aside, I think what Cindy represents is what lures fans in. She's the bratty, popular girl / high school cheerleader type everyone lusts after, but very few have the courage or the ability to score with. She's also the forbidden fruit in a relationship with a friend, prone to dragging Ian down if Ian is on the go-getter path. As a bonus, Ian can also deny Axel - the second biggest douchebag in the game - the opportunity to have his way with her.
Overall, Eva Kiss did a fine job writing her from the start. It seems everything has been a slam dunk so far. There have been no significant hiccups or rewrites for Cindy compared to some of the other characters. I suspect the challenging part for the dev will be how to write her once she's in a relationship with Ian because the whole dynamic between them will change.