It's a clash of 'normal' and 'comic' expectations. Like, when someone's braindead, it's usual to turn them off. Unless you're a superhero, in which case, superheroes assemble to help you.

I think it's possible to interpret her anger as "Tony's got a plan to fix himself; why didn't he have a way to fix Happy?" Like, she's not blaming Tony for cutting the life support so much as blaming him for arranging to save his own life, but not Happy's. Seeing him a vegetable must have brought back painful memories of Happy, and I'm not surprised that she had a little episode.

Also, when they slept together, she thought it was the last time she'd ever see him alive, but he knew, vaguely, that he had a plan to come back.