Jonah Lehrer explains why it is not for reasons you may think.
Belief, specifically your mindset, whether you believe you have a certain amount of intelligence and cannot do much to change it, or believe you can learn and improve at anything given the time or energy, has everything to do with your true capacity to learn.
Lehrer summarizes with some advice for parents:
The problem with praising kids for their innate intelligence — the “smart” compliment — is that it misrepresents the psychological reality of education. It encourages kids to avoid the most useful kind of learning activities, which is when we learn from our mistakes.
That’s probably as true with inverse as well – insults about innate intelligence – telling a child he or she is “stupid” or is a “dumb ass”.