Clearly it has been Bantam's goal to capture younger readers. From that perspective, Tommy Patterson is not a bad choice as the artist chosen for the job. TPs style is cartoonish and childish. All characters are rendered younger and more alike compared to the TV series. The younger characters is given more space, suddenly Sansa start seeming like an interesting character.
What also makes mature readers keep their distance is the poor depicting of human emotions. From the artwork it can be hard to tell if a character is truely annoyed or angry. As you read through the book you start fantasising what this could have been with an artist that suits the adult nature of the original story. Brian Bolland perhaps?