I think many people hate her because they are unwilling or unable to look beyond her arrogance.
Vivienne isn't a person you can like. You can respect her, you can appreciate her for several things, but you can't really like her. Meanwhile, she and my main Inquisitor have disagreements about mages and magic but she has good arguments for her position, which is why I don't dislike her for having them. She's the kind of person I can imagine having some interesting debates with. Also, she has good advice on leadership. Yes, she may use the Inquisition to further her own personal ends, but hey, so do I. Aspiring to power is not, by itself, a detestable trait. In fact, I appreciate her for being ambitious.
As odd as that may sound: I may not like her, but I'm a fan. She's one of those rare types: an ideological foil I can respect, and if she becomes Divine, yes, the Circles are reinstated but the mages' situation is still considerably improved from the pre-war state of things.
I still don't think she has any good arguments for her position. Well that's not entirely true, I still believe the Circles should be like the Tevinter Circles where they are basically the prestige universities instead of the "Alternative Schools for those who don't play well with others" Circles of the south.
She tells my Mage "Oh yeah when mages are free it's the normals that suffer, normals fear magic because some mage did this and that to this person with magic", When she herself uses magic and potentially kills a "normal" because of some supposed insult my inquisitor "felt"?
Honestly everything she has said to me in my plays has always been contradicting to her actions.
Example:
Says that mages killing normal people causes fear and is intolerable yet she freezes a guy(a noble at that) and possibly kills him because "he was rude". Yeah that's not abuse of Mage power right there yeah?
Then there is the whole deal with her having a mansion living outside of the tower, having a relationship with a Duke openly, and suspiciously killing said Duke(unclear?) but disagrees with you and even the tone of her voice changes towards you when you mention mages have the same freedom she has.
But all that is okay cause what? Cause she's right about the rebellion being a waste? I still think she is a viper who saw an chance to achieve the status and power she wanted so she took it by joining the Inquisition.