There has been much debate about your statement. Macbeth should be a tragic hero - meaning that he falls from a noble state to one of despair and/or destruction. Macbeth doesn't seem to feel remorse for his actions, though, and doesn't acknowledge his mistakes as most tragic heroes do. However, Act I supports your statement the best.
In Act I, we see Duncan bestowing honors on Macbeth for his valor in battle and loyalty to the king. Lady Macbeth herself says that, though Macbeth is ambitious, he lacks the treachery needed to achieve the throne. Even as Lady Macbeth begins making plans to murder Duncan, Macbeth is hesitating. He knows that Duncan esteems him greatly, and has recently commended him for his loyalty. However, as Macbeth progresses, he gets more and more consumed with power and ambition. Saying that he is foolish is perhaps underestimating his ambition, but he does make bad decisions that lead to his fall.