What does Holden telling Ernest Morrow's mother lies about her son rather than telling her how he often tries to hurt people by slapping them with a wet towel reflect about Holden's character?
Holden genuinely believes his own lies thinking that Ernest is actually a swell guy.
Holden, although not a big fan of Ernest, felt that Mrs. Morrow was too nice but not too sharp in the brains department; rather than ruin her outlook on her son, he thought it better to make Ernest out to be a shy and modest guy.
Holden views Mrs. Morrow as a mother he wishes he had; he thinks of her as a weak individual, like the other women in his life, who couldn't handle the truth about her son.
Holden is selfish, never thinking about others' feelings; Holden's deviousness is just one of his many ways that he tries to take revenge on the people whom he views as phonies and morons.
Holden enjoys telling lies about others, even if it makes someone he views as a jerk look like a good guy,