Sally's different to Holden in that she pretty much accepts life as it is. Breathtakingly shallow and vacuous, she sails through life without a care in the world, unable or unwilling to recognize its darker side. It's difficult to imagine a more contrasting world-view than Holden's. He's acutely aware—too much so, one might argue—of the numerous imperfections of this world and everyone and everything in it. To him, Sally's just the latest in a long, seemingly never-ending line of "phonies," those intensely annoying people who populate Holden's life at every twist and turn.
Having said all that, the fact that Sally's much more grounded than Holden makes her more mature in key respects. For instance, when Holden comes up with a deluded fantasy about their getting married and living together in a remote log cabin, it's Sally who's the grown-up, the one who points out the shortcomings of this crazy scheme. Far from being "phony," Sally's actually behaving like a responsible adult, and it's high time, we might think, that Holden took a leaf out of her book.