USMA looks at your class standing. That is what counts, not GPA, or a particular score in any class. On the other hand, you certainly don't want C's in classes when your competition for an admissions spot has nothing less than a B for example. It's all relative. If you are awesome in all categories, but happen to have a C in one class, I wouldn't wory about it.
Keep in mind, getting into West Point is about wining an admissions spot against the others who are seeking to get that same appointment. It's a competition. Much depends on who that is. Most candidates for service academies have very solid résumés academically and otherwise. They have that goal, and are driven. Thus, you want to do everything you can, stay focussed, and avoid getting chinks in your armor. On the other hand, noone is perfect, and most candidates will have a slip up in a class, or a low score on a section of the ACT.... If a C in an AP class is the worst of it for you I wouldn't fret things too much.