Nope. Unless a very recent retcon happened, Evelynn is an *extremely* old being. Thousands of years old at minimum but she's probably vastly older than that even. The above quoted is catastrophically wrong.
That being said, I don't agree with the other guy that TheCount doesn't use Evelynn because she's super old. I think it's because typically TheCount has his female characters get dominated. Evelynn is not the type to be dominated but rather the one to do the dominating. Her old moniker was literally 'The widowmaker' and she wore even more blatant dominatrix stuff originally. That theme was toned down a bit but she's still the dominant type. Very much so.
The tl;dr = Evelynn doesn't fit TheCount's themes or style. He could use her anyway but that would be like using Sona for a vid and having her actually talk.
Evelyn in canon Runeterra lore is a millenia-old demonic being. I’m not 100% on K/DA lore, but, being an alternate universe, it’s entirely possible that her age is that of a regular human.
In any case, I don’t see why Evelyn’s canon personality would be a deterrent; it’s not like Kai’Sa, Ahri, or Akali, for instance, would let themselves be taken advantage of any more than she would. Or any of the
Dead or Alive girls, I’m guessing. No matter how you look at it, The Count is just having stuff happen to these women because he wants to, not because it “fits their character” to get raped.
I agree with the part about domination and you can't apply the Rape tag to her

So maybe these are the reasons why he didn't make content with her.
The hell do you even mean by that? That she wouldn’t let herself get raped? That she’d consent to whatever, so technically you couldn’t rape her? I have serious doubts regarding the latter proposition, but, like I said above, The Count has the power to do whatever he wants.