i dont really see how this was sad? sure she died but became a snowwoman and can see kuuko now.
Kuuko had to one-sidedly watch her the entire time she was growing up and feeling some guilt each time, and then at the end she was indirectly responsible for her death, and to top it off Yukime was HAPPY that she DIED because she got to see Kuuko again.
She was so obsessed with Kuuko and also air-headed enough that she didn't even care that she died from doing all that.
Least that's how I interpreted it. (gotta look at it from Kuuko's point of view) If anyone has a better one feel free to trump me.