Why is Mackenzie important in Dork Diaries?

In the "Dork Diaries" book series by Rachel Renée Russell, Mackenzie Hollister doesn't play a significant role in the story unlike the other characters. Rachel Renée Russell, the creator of the "Dork Diaries" book series, has centered the storylines primarily on the main character, Nikki Maxwell, and her challenges navigating middle school.