So a couple things I've been able to come up with this whole weekend that this has been a thing:
/1. The "makeup argument" doesn't really hold water when basically all of Riot's female champions have makeup on. Even Illaoi, the champion of body diversity, has perfect eyebrows, mascara, and eyeliner (not to mention another wild dweller Nidalee. Hell, even Annie has eye makeup). Why? Because it tends to make them look nice. If you have a problem with that, then that's something to bring up with Riot's design structure as a whole, not something particular to Kai'Sa.
/2. Yes, you could definitely describe Kai'Sa's proportions as generous. BUT, to hold to the entirely opposite opinion that because she was living in the wild means she should be sticks 'n bones thin is entirely bogus. Like, seriously, go and look up just about any crossfit woman. Do they look sickly or starving? It's important to remember, Kai'Sa wasn't just surviving in the caves, she was thriving. She was a huntress, and a damn good one. She was more than capable of taking down void creatures many times her size, and certainly anything else that wandered down there from the surface. Now, could she be more (for lack of a better term) "flat chested"? Yes, of course. But, being as this is a fantasy game, I'm not going to fault the devs for wanting to make her more fantastical.
So in sum: she could definitely be less.....voluptuous? But to argue that she shouldn't be at all is a farce.
/3. As recently (re)pointed out by Placentas on this thread, Kai'Sa was never actually living in the Void. She travels through it occasionally (which is her ult in game), but during that time, her suit seals up, protecting her from any energies that could effect her. As for why she was not zombie or monsterfied in the first place, that is explained in her story. She was incredibly lucky that a) the first void creature she ran into was relatively small, b) she was able to kill it, and c) that the void creature bonded with her skin forming a suit, rather than dissolving her or whatever. MacGuffiny? Sure. But not unexplained. Riot made sure to write themselves in out with the Void lore update:
Though it has bred many of them, no two horrors of the Void are exactly the same. Their otherworldly forms are as varied as they are terrifying.
Basically what the amounts to is that the Void can do anything Riot wants it to do Again, a bit MacGuffiny (am I using that word right)? Again, sure, but you can't say Riot didn't at least properly set it up for themselves.
So that's basically all my thoughts on this matter. If not obvious, I don't agree with the opinion that Kai'Sa is "oversexualized", or even "sexualized", for the reasons above.