It's true-- if Derp was human, she would probably be considered handicapped. I hope that none of you are bothered that I make fun of her-- truly, I mean no negativity by it.
I don't think it's bad you make fun of her. I doubt any chicken would understand what derp is and get offended, besides, if a chicken is derpy, she probably would never be able to understand it anyways XD. We have a pullet that, while she's not quite derpy, she isn't the brightest chicken out there. We joke that the little hamster living in her brain fell off its wheel and now it's running in circles when she gets that look of extreme concentration on her face.