Better memory does not automatically equal better grades. It differs from subject to subject, as well as depending on at which level you are studying.

For example, take a science subject. In lower level courses (up to high school level), much will depend on your correct regurgitation/recollection of facts. As you progress to higher (college/university) courses, more rational thought and logic has to be applied. At this stage, having a good memory alone will not ensure success.