I think Holden's comment about Sally is similar to what a child might say when he doesn't get what he wants - he rationalizes that the object of his desire wasn't that great anyway. Holden's irrationality by this point is becoming more and more noticeable. He has tried to share with Sally the basis of his depression and impulsively asked her to run away with him to Massachusetts and Vermont. His agitation and the precipitousness and impracticality of his request frighten her, and she turns him down, and he is hurt. His reaction is to put her down in return, by speaking cruelly to her, and by rationalizing in his own mind that, despite what he might have thought a little while ago, she wouldn't have been so great to go with ("she wouldn't have been anybody to go with") anyway (Chapter 17).