People are complaining that Kai'Sa isn't corrupted by the Void enough
Now, granted, I understand that the reason people are disappointed is because she popped out of the Void after years on end with her face looking like Scarlett Johansson's, while Kassadin and Malzahar got royally fucked up by it physically after being exposed to it for a far shorter time in comparison. People are complaining that she doesn't have many Void-y traits, which would be like what Voidborn Ilaoi has, and that she basically just turned out of the Void after years of exposure without so much as a scratch.
However, the Void's influence on her goes way deeper than just physical appearance or attributes. Take a moment to remember how her "suit" basically works from her reveal page.
> Kai’Sa endured through sheer will and a symbiotic second skin adapted from a living Void creature.
Now, the definition of symbiosis: Interaction between two different organisms living in close physical association, typically to the advantage of both.
Going off her reveal's short blurb and this definitions, her "suit" is literally a living Void monster that has latched onto her a la a Kamui from Kill la Kill, and both Kai'Sa and this monster both rely on the other to stay alive at all in a place as hostile and terrifying as the Void. Both of them have something to gain from the other, and that is assuring their own survival. And that's not even bringing up how Kai'Sa probably had her body altered in some crazy way to accommodate this Voidling, possibly far worse than anything that could've happened to Kass or Malz. Altering your physical appearance isn't the only way something as bizarre and eldritch as the Void can affect or get to you.
All in all though, considering that we literally have three sentences of her lore out so far, I'm pretty sure that the actual full lore will explain what happened to Kai'Sa far better than what this borderline crack theory could ever hope to accomplish. Then again, this entire post was made because I'm just frustrated by how everyone seems to be crying foul for not "designing her right," and this is only my half-assed theory on why she is the way she is and as such should be taken with a grain of salt.
But seriously though, how can you say she's not designed right? I mean come on, that Voidling attached to her is literally Samus' and Pharah's suits rolled into a Kamui and she has the Needlers from Halo, what else could you want? She's designed like an absolute badass.