Complexity is a human construct. I am not saying God is 'simple' by any definition of the word that I am aware of. He is so far beyond our ability to truly comprehend that those two words have no meaning. I would say that he is not 'needlessly complex'. In that sense, and in that sense only, I would agree that he is simple. There is nothing that can be added or taken away. In that sense, yes, he could be called simple.