She is a professional, a paid performer. She decided on this career, she decided to take that risk.

Sure, if she was an ordinary kid, and this was a high school concert, or a school recital or something I agree she shouldn't be booed, but she is the one putting herself out there as a performer - as a singer. How should people express disappointment in a professional's performance?

By the way, I doubt her self-esteem has suffered one iota. And I do not mean that in any negative way - she still got paid for the gig, right? Maybe she'll invest in some training.