Small mindedness is not the worst of things. Smallness in faith is the worst thing. Small mindedness keeps us from seeing the grandeur, the expanse, and the plan of God. Small in faith means we see little more than what we can pull off ourselves.
Jesus had said that he gave his followers authority to cast out demons and cure diseases. The issue is not whether they were capable or had the power. They simply had very small faith. Now I'm not sure if the problem was caused by Jesus' absence, the seriousness of the boy's condition, the hardness of their hearts, or all three combined. However, they lost faith that Jesus would do what he said he would do.
So many of us lost that faith. Jesus reminds us that it doesn't take a lot—just a mustard seed amount. If that's there, then incubated with faith it becomes powerful and mighty.