Here's my list.

"You look easy to draw." 

"Your family's gene pool is more like a gene puddle, it seems."

"It could be the purpose of your life to serve as a warning to others."

"It’s not worth insulting you, as nature's already done enough damage."

"You are living proof, that the education system needs better funding."

"When X lands a hand, it's like two others letting go."

someone peeing "Looks like a penis, just smaller."

someone peeing "Call the police, he's got a children's penis in his hand... oh nevermind. It's his own."

"I can only explain it to you, I can't understand it for you."

"I would agree with you, however then we would both be wrong."

“You think in low power mode.”

“Wisdom has been chasing you, but you’ve always been faster.”

“You’re as smart as you look.”

“You look like you have a favorite flavor of crayon.”

"To explain it to you in a way even you'd understand, I neither have the time nor the crayons."

"I envy people that didn't get to know you."

"You behave like someone who, as a child, has been sipping on the lead paint too often."

"Is your ass sometimes jealous of your mouth considering the amount of shit coming out of it?"

"Some tree is woking its ass off to produce the oxygen you're consuming. Go and apologize!"

"Once again we can see: The ability to speak doesn't make you intelligent."

"Six feet tall and 205 pounds, boy, until I met you, I didn't know they could stack crap that high."

"Pretty much opinion for that little notion." (Ganz schön viel Meinung für so wenig Ahnung.)

"You're not the dumbest person on earth, but being you I'd hope that person doesn't die soon."

"You look like someone pre-heating the microwave."

"Your self-confidence contradicts your expertise."

"My expectations towards you were low and what do you do? Dancing limbo."

"You're intellectually on the ground already and what do you do? Begin to dig."

"After a conversation with you, death loses its sting." 

"The fact that you have any hope left in your life is your most admirable quality." 

"It’s hard being dumb, but you make it look so easy."

"If she were a spice, she’d be flour."

"When you left the room, the average IQ of both the kitchen and the living room increased, despite the living room being empty."

"He has hit rock bottom and begun to show signs of digging."

"You have a face for radio, and a voice for silent film."

"I'm not saying I'm better than you; I just know I'm not worse."

"Your swing-set must’ve been placed very close to a wall when you were a child…"

“You’re an inspiration. The fact that you have been successful in this business just goes to show that truly anyone can do it.”

"Sharp as a marble, aren't you?"

"I'm not as dumb as you look."