Yoko's heart is not the issue, but rather how the writers decided to make her the Doomchick. It really isn't her fault Kittan jump-kissed her out of nowhere. He did that to himself. And what Yoko did with herself post-timeskip and even in the after-credits shots shows with whom her heart truly lay.
Allegedly, GAINAX mentions on its TTGL page that Yoko helped Simon lose his virginity. Given Simon's initial-- and entirely understandable-- fascination with her, I'd call that a class play.
Yoko's heart is not the issue, but rather how the writers decided to make her the Doomchick. It real