One cool comeback for any situation is, "Oh, this coming from someone who fell down the ugly tree and hit every branch on the way down?" Another comeback is, "I don't know what makes you stupid, but it really works!"
Other comebacks include, "Does everyone visualize duct tape over your mouth so early in the conversation?" and "Oh, I'm sorry, was I supposed to be offended? The only thing offending me is your face." Some comebacks are designed for specific occasions, such as for when clothing choices are insulted: "How come I always wear these clothes? How come you always wear that mask?"