They do make fairly frequent references to D&D which are intentionally skewered though they do show that the writers do have some experience. Other references throughout the shows which come to mind readily include the Billy's father having fuzzy d20's in his car, semi-frequent references to d10's and "Dungeons and Dinobanoids" as well as I'm sure a handful I'm missing.
I'd enjoy the show more if it had less Billy and more Mandy. The forced stupidy of the Billy-centric episodes just make my head hurt...which may be the point. Mandy's misanthropy, on the other hand, always makes me smile.
That show is odd, but the Wrath of the Spider Queen was definitely a high point. The dark elves and their constant infighting, The evil carton of milk, the Hobbit reference (Look! Billy's brought the Eagles!).