Why is it that in Pokemon Dawn gets called DD by her old friends?

Only Kenny calls her that, because it's supposed to be an insult. (It stands for Devilish Demon.) Also 'dawn' means night, or extremely early in the morning. People say that dawn is a time the devil likes, and DD also means "Devil's Dwelling."

it might not be that because in the episode Hot Springing A Leak, her best childhood friend, Leona, calls her it. Ash says to Leona, "Why do all of Dawn's friends call her DD?" and Leona says, "You don't know? Well, you see--" and she is cut off because Dawn dunks her under the water before she can say anything else.