Well first off, we can't assume anything and hold it as the truth. For all we know, Karin could almost be like a god mother to Sarada. Aside from the glasses, Sarada takes after her father the most in looks and personality. If she never had glasses, no one would have bothered to say "Hey look, she looks like Karin".
However, let's just think about the impact on the plot if Karin turns out to be her mother... or if Sarada just feels this is the case, but Sakura really is her mother despite the glasses and picture.
Sarada would feel pain and betrayal, which might push her to awaken sharingan. Being a girl, she has less of a chance of gaining it, but it has happened and is possible. So, this would be the only way for her to feel enough pain in this era of Naruto. She would then seek out Karin or try to find out more about her, which possibly would lead her to Orochimaru because I'm sure Karin and gang went off with him after the war if he survived. Orochimaru would then possibly become the main villain once again. To make it further twisted, Sakura indeed turns out to be Sarada's mother and Sarada would feel pain once again for betraying her family and also for being lied to by Orochimaru. This will either unlock more of the sharingan or maybe some special ability of her own.
However, let's also consider how long Kishi wants to keep Naruto going. It seems to me he wants to do this last Gaiden and Bolt movie.. maybe one last game and be done with it so he can work on other things..from what he expressed with the ending of Naruto at least. Is he really going to pull all of this just so he has a base for a whole 3rd section of both the manga and anime?
Here are things to consider.
1) Why does Sakura even have the kid if it's not hers? Why is she super loving towards the girl? No one seems to question that Sarada is Sakura's daughter.
2) Sasuke did in fact have time to do the nasty with Sakura. He was their for The Last movie and would have time before he left to impregnate Sakura and possibly marry her before he set out again.
3) We don't know what exactly happened between The Last and Boruto Gaiden. We can't assume that Sasuke hasn't been back twice before Sarada was even born.
4) Again with the appearance. Had Kishi not given her glasses, I'm sure no one would question that she is Sasuke's and Sakura's child. She looks and acts like a typical Uchiha/Sasuke and I guess you can say she looks like her grandmother too. And she also looks enough like Sakura where you wouldn't question the relation. We can't sit here and assume that just because she has glasses and just because she found a picture covered up.. that means that Karin has to be her mother. It could be to just create enough tension and shit between her and her parents. (like I said in my own short theory)
I guess all we can really do is just wait for the next following chapters.. whenever that will be.
_@: 
is this mexican soap opera? Really? Kishi really got into trolling...I wouldn't be surprised is Sarada turns out to be the daughter of some other kunoichi though everyone now thinks it's obvious who is. At the beginning I thought Sarada's story is the new version of the same Naruto's childhood story, but now...
