The main character is not white. She is described as being brown. (At least half Mexican though not all her lineage is given in detail).
As far as I can tell, the majority of named, principle characters in this book are either of Mexican descent (brown), or black, or native american. (How they are depicted in the text is a whole other topic, which I won't get into here).
Furthermore, Grace does not have frilly pretty ivy tattoos, she has audacious tattoos of things like portraits of people and saints, and phrases from hotrodding; working on old hotrods is her livelyhood. The book specifically states that she's not into flowery tatoos.
The cover is so wrong with regard to the tatoos, Brian suggested maybe Tor just pulled an illustration they already had and put it on the cover. I don't know. I doubt it.
Earlier at Millenicon I also had a conversation with Jim Hines about this sort of thing. Jim has a principal character in The Stepsister Scheme who is not white, but on the cover illustration that's not illustrated dramatically enough for it to necessarily be obvious, though it is supported by her clothing. He commented that he wonders if he should have pushed harder about that, because the character's color really should be darker.
Then today oyceter posted about the extreme whitewashing of Angel's Blood, by Nalini Singh (the main character is half-Moroccan and should have "dark gold" skin, but the white hair is canon). I find myself wondering if there are other examples I've never noticed or come across, and what they are.
With regard to Grace part of the problem, of course, is that there's no single archetype of "Mexican woman", and while I am frustrated that the woman on the cover doesn't look like, say, my sister-in-law, who lives in Arizona and is of Mexican descent, there are probably at least mixed-race Mexican women she could be an (idealized) portrait of. But still, given that range, there's a choice. Go with something ambiguous, or make her skin brown and her features Mexican (rounder nose... hmm... the difference is hard to describe. Like... um, like this or this). What she looks like, to me, is slightly Spanish. It's not strange for this to seem acceptable, since thanks to Hollywood we often see spanish actresses (see Paz Vega) play mexican characters. One of our most acclaimed Mexican Actresses, Dolores del Rio, in fact came directly from a family of Spanish Basque descent, though she was born in Mexico. And then our sense of what Spanish women look like even gets defrayed as they are played by people like Catherine Zeta Jones, who is Welsh. So what bothers me there is not just a skin-color thing, it's a features thing. Proportions.
And now, if I could really draw, I would start organizing workshops at sf cons about how to draw characters of different racial backgrounds and mixes. I might do that anyway. I just have to study it myself first, or find some artists to support it.