That's just one layer because is not just any other guy. It's your friend who is taller, more handsome and used to be way more successful with girls than you in your teens. And Cindy kind of embodies the trope of the hot popular cheerleader that paid no attention to Ian and was way out of his league back in the day.I'd guess it's the "stealing other guy's girl" factor, since it's the only storyline like that.
So "stealing" Cindy has the forbidden fruit appeal with an extra of betrayal of a close friend. And it also works as validation for Ian that despite everything (his awkard teens, Gillian's break up, struggling writing career and so on) he's now better than Wade and he can get a girl like Cindy. I don't think it's about Cindy as an individual and more about what she represents, kind of the platinum trophy Ian gets for being more confident, working at the gym and getting this career on the right track.
And Cindy as a character is the insatisfaction of someone with a normal boring life when confronted with the (fake or not) appeal of the exciting and successful social media stardom. I find her refreshing compared to most other LI (on this game and others) that she provokes love and hate reactions not just based on her looks and attractiveness but on her contradicting personality. She has obvious flaws and it kind of makes her more real as a character and enhaces the guilty pleasure vibes on her path.