She's supposed to look like Karin, even if this is all a red herring and she is Sakura's biological kid - she's supposed to look like Karin for the storyline.
Personally when I first saw Salad in the manga I assumed she was Karin's kid. That was my initial thought.
But lets assume for a moment that she is Karin Uzumaki's daughter.
... still with me?
She doesn't have to look 100% like Karin - features that don't match up can be accounted by the fact that Sasuke makes up the other half of her DNA.
Glasses aside, she reminds me a lot of a chibi-Sasuke/Itachi and Mikoto Uchiha. *shrug*
Besides the anime has deviated from the manga and Kishi's original work/designs in so many ways it would be too tedious for me to list them. I don't think using the anime as "evidence" to back claims that Sakura is Salad's biological mother, or that Karin Uzumaki isn't is all too clever.