I enjoy people being frustrated with or suspicious of Ashley's goodness. I'm not sure I would have ever discovered that if not for readers being highly suspicious of her, so many thanks to the more cynical among you who assumed from the beginning that Ashley must be up to something.
I intend to add emphasis to why Pandora can't mark Ashley in the next comic, but this is a Friday comic, so in the interest of not getting asked about it a lot over the weekend, I will say this (potentially VERY minor spoilers):
The issue here is that Ashley's fantasies are primarily about transforming other people in ways comparable to Tedd snapping and running around Moperville transforming people without restraint. She knows full well that that would be wrong, and that creates a conflict great enough to get in the way of giving her spells via magic marking.
In other words, she's just short of being a gloriously perfect candidate for what Pandora wants.