As previosly implied, Ian's comeback's are not the best nor effective (some aren't even comebacks) but, are humorous because of this.
(Anthony: italic Ian: normal)
"I'm having a really hard time opening this barbecue sauce."
"Hey, I'm Anthony. I don't know how to open things. That's why I'm dumb."
"Ian, your hair looks like it was invented 50 years ago in the 70's."
"50 years ago in the 70's? Isn't that when I f**ked your mom?"
"Ian, your ears look like they belong on a chimpanzee."
"I use taco-tongue, like, 3 times a day."
"To do ... what, exactly?"
"It involves your MOTHER, ha-ha-ha!"
"(Contemplating look) Okay."
"Ian's Mom is gonna be really pissed if she watches any of these episodes, I really hope she doesn't watch any of these."
"My mouth's full of bones."
"Hey, you have a moustache on your head."
"No I don't."
"Hey, don't talk with food in your mouth."
"Hey, don't talk with a dick in your mouth."
"What do you think a root fruit would look like?"
"Do you always spread your legs like that?"
"Only for your mom."
"Dammit, you're always sitting there eating while I'm finding the questions. That's why you're always done way before me."
"Or maybe because you're a slow eater. And a wiener!"
"Ew, that got on my face."
"That's what your dad said to your milkman."
"Why don't you put your ribs down and start filming something."
"Why don't you put your face down and stop being ugly."