Mhairi is a character that is very flat but a lot of people seem to have attached themselves to her.
It's probably the same phenomenon you see in the Inquisition forums. We know next-to-nothing about two-thirds of the characters in Inquisition, but there are hundreds of pages about them dissecting their personalities, and expressing adoration for each of them. As blank slates, they can be anything the player wants to be, and the danger of releasing their names and faces so early is that by the time the game is out their fans have a fully-fledged personality made for them built by projected desires.
And whenever the final result doesn't match the image in the player's head, the result is a weird sense of betrayal.
Same for Awakening, but then Mhairi featured large in the promotion, and got killed off at the end of the intro. Having had the good sense to die, she remains a blank slate, and is still the character everyone wanted her to be. 
That, and Alix Wilton Regan is an awesome voice actress.