A funny comeback to use on people who talk behind others' backs is, "If you must be two-faced, could you at least make one of them attractive?" A similar comeback is to tell the person, "I won't waste any energy giving you a nasty look because it appears that you already have one." A funny response to someone who says something idiotic is, "They say brains aren't everything. In fact, they're nothing in your case."
If someone makes a childish insult, a funny comeback is, "Wow, nice one! The last time I heard that insult, I cried so hard I fell off my tricycle!" This insinuates that the insult is so immature that the only people who find it hurtful are children too young to ride bicycles. The comeback is a variation on the quote, "That's so funny. The last time I heard that, I laughed so hard I fell off my dinosaur," from the film "Step Brothers."
A simple yet effective comeback to an insult is, "I've been called worse by better." This comeback is so effective because it manages to put down both the original insult and the person who said it at the same time. On one level, it suggests that the insult isn't that powerful because there are worse things the insulted person has heard. On another level, it passive aggressively lets the insult-giver know the insulted person doesn't take her seriously, as she has interacted with "better" people more worth her time.