She's not a better heroine and Sakura has a normal "background".
Here's an analysis I wrote a few weeks ago
One big difference between Karin and Sakura is Karin's background in comparison to Sakura. Karin's entire village was burned to the ground and every villager murdered, including her family. As the sole survivor, who knows how long she wandered around before Orochimaru found her and took her in. He's the worst possible person to provide with a sense of purpse and identity, but he does. He groomed her from a young child, and of course she clings to the perceived generosity, being too young or naive to know any better. She was subjected to experimentation and all manners of cruel and painful shit. She grew up in Orochimaru's labs, being subjected to all manner of depraved behavior and sights. Perhaps, in this way, seeing Sasuke's smile again may have reminded her of some sort of light, probably one of the few true acts of kindness she can remember receiving. So yeah, Karin's horny and aggresive, but considering the way she grew up, I think "I just wanted to see you smile again" is a pretty rare kindness and humanity for one to still possess.
Meanwhile, we have Sakura. Both parents alive and loving. Multitudes of friends. One of her friend's only desire in their friendship was to see her truly blossom and reach her fullest potential. Two of her friends, as well as her first friend, are willing to die for her. Had a normal, trauma free childhood. Has people constantly trying to protect her. Trained by two legendary ninjas. Lives in one of the strongest villages surrounded by people who care about each other. She has and had more than 95% of the characters in this manga, and she displays less backbone, self esteem, and substance than a side character who had virtually none of the things she was blessed to have in her life. My point being that Karin's depravity can be understood through her depraved upbringing. Its not an excuse, but we don't have to expect much from her. Meanwhile, Sakura is the heroine of the series and is content with being treated like garbage and her highest aspiration is getting this guy that makes her feel like shit to be with her, and can't even drudge up any substantial or intelligent thoughts about his person or communicate anything of depth about her feelings .
Another difference is that Karin is made of tougher stuff than Sakura. Perhaps its why Sakura looks just as pathetic in comparison. While Karin didn't shed a tear when Sasuke tried to kill her, Sakura breaks down. While Sakura's world comes to a screeching halt when Sasuke doesn't notice her in the face of one of the strongest ninja in the world, Karin wouldn't have batted an eye. She may be horny, but Sasuke's brusque or otherwise plain manner of expressing himself doesn't tear away at her self esteem the way it does Sakura. Even normal things, such as telling her she can't come with him on his redemption journey, send her spiraling into self pity and angst.
This isn't some anti Sakura, pro-Karin thing, but it is sad that the main heroine and girl in the pairing is really no different than the character they love to bash in comparison, and practically anything that can be said about one doubles back on the other. Like the manga shows, Sakura's sole advantage and relevance to Sasuke being she was on Team 7, and any Sasuke fangirl on his team would have gotten the D just the same. Sad, but hilarious.
