Of course they do. And you know how they do it? They use average ratings for shows their in, ratings for shows they guest star in, "draw" at special events, relative amount of web traffic /fansites related to the individual, twitter followers, facebook likes, focus groups, etc.
And obviously I don't have access to all that information, but the factors I do have access to clearly show Ashley is at least on par with Demi.
I wouldn't dispute that Ashley is on par with Demi in fact, it wouldn't surprise me if Ashley was more popular than Demi. She may have even been more popular than Debby, Selena and Miley at one time. That's not what we're debating here. The only point I'm trying to make is that Ashley was never . . . at least officially . . . DC's "it" girl or leading lady - whatever you prefer to call it.
For me personally I prefer Ashley to Demi and I would guess you would also but that doesn't change anything in the history of DC's leading ladies.