This is an interesting question. I would say it is neither. Oedipus is not stupid (in fact, I would say that he is just the opposite), and Sophocles's play is not defective (I would say it is near perfect).
If you think about the play, Oedipus is an extremely able person. He figured out the riddle of the sphinx and he is the ruler of Thebes. The reason he has a hard time coming to certain conclusions is because everything is so far-fetched. If someone told you that you killed your father and married your mother, would you believe it? Moreover, all people have blind spots.
In addition, when it comes to the play, Sophocles has to do certain things. He needs to establish the setting, develop characters, build up the tension, and bring out a tragic ending. For these reasons, the play has to proceed at a certain pace. There is nothing defective. In fact, most peole see it as a masterful work.