Mary Ann Glendon is not a Vatican official. She was U.S. Ambassador TO the Vatican. In other words, she was appointed BY the U.S. and represented the U.S. to the Vatican, and not the other way around.
Also, even if she were a Vatican ambassador, that's not against Catholic doctrine. Ambassadors aren't necessarily priests or bishops, i.e. ordained clergy. Ambassador isn't a priestly position, it's a diplomatic position.
However, I think nuncios (Vatican ambassadors) are usually archbishops, because they play a role of being liaisons between the national bishops' conference and the government of whatever country. Also they play a role in recommending bishop appointments. They nominate them, so to speak.
Thank you for helping me understand! :-) I admit to being lost sometimes in reference to many Catholic practices. Thank you for taking the time to explain.