The bell curve of IQ and being stupid probably don't have much to do with each other.

Think of stupidity as the consequences of interacting with ones environment with negative outcomes. If you have a simple environment with few negative outcomes, then even someone with a 80 IQ may not be considered stupid. But if your environment rapidly grows more complex and the amount of thinking you have to do for positive outcomes increases then even someone with a 110 IQ may find themselves quickly in trouble.