What does Holden mean when he says, "Mothers are all slightly insane"?

As the main character in J.D. Salinger’s Catcher in the Rye, Holden Caulfield is in an emotional and psychological downward spiral throughout the book. As we listen to his first-person narration we begin to realize that we cannot take everything he says at face value.

Holden makes quite a few value judgments over the course of the story. Sometimes these judgments are valid critiques of life from an adolescent point of view. Sometimes, however, the judgments have an ironic meaning—Holden doesn’t realize that he is revealing his own character weakness.

In chapter 8 (page 55 in my edition) Holden is riding the train home. When an attractive older woman sits next to him, he learns that she is the mother of a Pencey student named Ernest Morrow that he does not like. When he says “Mothers are all slightly insane,” he is commenting on a mother’s tendency to see the best, and ignore the worst, in their own children.

Previous to this statement, Ernest’s mother reveals that she is concerned about Ernest because he is too “sensitive” to “mix” well with others. This draws a critical comment from Holden (keep in mind that he says this to the reader, but not to the mother):

Sensitive. That killed me. That guy Morrow was about as sensitive as a toilet seat.

The truth about Holden is that he is actually the one who is slightly insane. He is suffering psychologically and will soon reach a breaking point. Late in the story he reveals that he has been in treatment, and, although he does not give details, we know he was struggling emotionally. Throughout the novel he has criticized those around him, and the revelation that he has had a breakdown of some sort brings into question all of his judgments about others.