If Amy were mentally ill, she wouldn't have been okayed for the show
I don't think anyone can watch how amy has behaved and believe it's the behavuour of someone mentally well or emotionally stable. She clearly didn't present like that before going on and as said, I doubt anyone would have been able to predict this.
But deciding she loved Curtis, he was her future, that he had taken her future from her and she had nothing, that they were going to get married and have babies, and to then have spend days sobbing, and have to be removed for counselling, when he'd not even asked her to be his girlfriend and had told her when she hinted to wait and see how it goes, is not the behaviour of a well adjusted, mentally well, emotionally stable person.
It simply isn't.