Simple minded to me means naive, ill-informed, unquestioning, lacking the inclination or the capacity for critical thinking. Such people adopt the the predigested beliefs and attitudes of those around them, rejecting all matter of available evidence contrary to these often baseless beliefs and they fail to take initiative to plan for their futures and they rarely commit to working with others to make life better for those who most need it.
Thus I suggest that the simple minded tend also to be likely also to be closed-minded. If you don’t care to, are unable to, or refuse to open your mind to alternative viewpoints because you are simple minded, then you are also closed-minded.