And she is a follower of Molag Bal (atleast I assume she still is) and her past is long and cold at heart. She believes that there is no way Mara would ever give her blessing between 2 people who would wed under the following of an evil dark lord which is understandable because during her time she probably helped Harkon sacrifice 1000 innocents in the name of Molag Bal all for the sake of Vampirism. So yeah she more likely would get smited by Mara rather than allow her to marry anyone.
Still though I think there should've been a "marriage in the name of Molag Bal" and not have Mara give her blessing. Who needs Mara anyway to get married?