Why do all of Dawn's childhood friends call her Dee-dee in the Pokemon Anime?

Answer 1

The reason why all of Dawn's childhood friends call her Dee Dee is revealed in DP161. She hates the name because of an incident that happened when they were all in Nursery School.

Dawn, Kenny and Leona had been put in charge of taking care of the Plusle and Minun at their school - feeding them, changing their water, and cleaning their living area.

One day, however, Dawn was chasing after the two Pokémon while Kenny and Leona were diligently attending to their duties. Her two childhood friends quickly got irritated by her fooling around, and told her to do her job. Dawn, finally catching Plusle and Minun in her arms, insists that she is helping, by keeping hold of them so that they don't get in the way of changing their food and cleaning. The Cheering Pokémon in her arms cry out in protest and struggle to get free, but Dawn scolds them, telling them that it isn't time for playing.

Kenny realized that the Pokémon don't like being handled in such a way, but Dawn insists that there's "no need to worry" - she's great friends with Plusle and Minun! The two Pokémon, finally at their limit, unleash an Electric attack to get her to let go. The attack works, and as Plusle and Minun scampered off, Dawn looks completely stunned, her hair sticking out on end and glittering with static electricity. Leona asks if Dawn is okay, to which Dawn weakly answers that she was. Kenny points out how cool Dawn's hair looks, with Leona noting that it's glittering. Building off of Leona's words, Kenny reasons that rather than Hikari (Dawn), she's Pikari (Dee Dee)! Leona approves of this nickname, and declares that that would be Dawn's name from now on, though Dawn herself doesn't look too pleased.

Later on, during storytime, Dawn's glittering hair is attracting quite a bit of attention from her classmates. In the middle of the story, Kenny suddenly stands up and shouts out Dawn's new nickname to the class. Their classmates begin whispering and laughing at the nickname, causing Dawn to start crying in humiliation.

All information was found on Bulbapedia http://bulbapedia.bulbagarden.net/wiki/DP161