With regard to her actions concerning Dorian's parents, she was only going by what apparently concerned parents were saying and asked her to do. She doesn't know any of the history lying behind his actions; merely that he had upped and left home without telling his parents his plans. So she thinks she is helping. I'll give her that one.
With regard to her disapproval over Dorian's friendship, I'm pretty sure she says that "we" are concerned and is not using the royal we but has been made spokesperson for the wider community. Bear in mind that Dorian later tells you that the blacksmith actually spat at him on first meeting him and if you ask Cassandra for advice she says not to trust Dorian, so in this instance Mother Giselle is not acting alone. I was angry with her but also at the other people involved for not having the courage to speak to me direct.
My biggest criticism of her is how she tries to fob you off with her watered down version of Chantry doctrine, presumably to mollify you if you seem unhappy with your title of Herald of Andraste. In particular my elf quizzed her about the Exalted March on the Dales, which she claims was not a proper Exalted March since only Orlais took part. We then go to the Exalted Plains and the accounts given there clearly shows it was considered an Exalted March, particularly the accounts of Sister Amity. I felt like dragging her over there and showing them to her.
Also after finding out it was not Andraste in the Fade, I suggested we ought to tell the people the truth and she had no trouble in rejecting the idea. However, once again, she was not alone in holding these views and you are fighting a losing battle convincing anyone about it; the best you can do when asked is simply to reply "no comment."
So on the whole I'd say that Mother Giselle isn't any worse than other members of the Chantry hierarchy and at least she does genuinely care about people.