Wearing makeup in general is haraam, but is a woman allowed to wear very very light face makeup just to cover up some bad acne or dark spots that are embarrassing for her, and what about when shes going to places like the haram or the Ahlulbayt in Karbala and najaf, is it okay to do so over there?

Salam alaikum. Anything that is counted as zeenat (beautification) and can be seen by others should be hidden from non mahrams and this is regardless of where a person is, whether in their hometown or in the Haram of the Ahlulbayt (as).
As for applying very light make up such that it cannot be seen by anyone and no one can recognize the person has applied something on, there is no problem.