The main character (and narrator) of Mattie, or Matilda, demonstrates honesty in two ways: by being a typical teen living in the post-revolutionary period in America and by giving avid examples of historical fiction due to descriptions of setting. Let's take the two examples each in turn.
Young teens, finally able to deal with the realistic world around them, are often brutally honest. This takes the form of antagonism between Mattie and her mother, Lucinda. Mattie is honest about not wanting to do her chores (while still knowing that she "needs" to do them). Mattie is honest about how she feels annoyed by her mother and the restrictions she puts on the family as they run the coffee house. In regards to Lucinda being the antagonist, Mattie says this:
[My mother] had given birth in the morning and cooked supper for ten that night.
For Lucinda, there is little compassion for her daughter as she struggles through her teen years. This lack of compassion and expectation of the highest form makes Mattie grow up fast enough to endure through the yellow fever and beyond.
Mattie is also honest about her feelings toward the first "important" death in the story: Mattie's friend, Polly. Mattie is absolutely shocked by this death; however, due to her duties at the coffee house, Mattie doesn't have ample time to grieve. Mattie speaks about this issue here:
“No. Absolutely not. I forbid it. You'll have nightmares."
"She was my friend! You must allow me. Why are you so horrid?"
As soon as the angry words were out of my mouth, I knew I had gone too far.
"Matilda!" Mother rose from her chair. "You are forbidden to speak to me in that tone! Apologize at once.
Most importantly, the teenage Mattie is honest about her conflict with the disease itself (especially as it poses a risk to her mother). Mattie's mother, Lucinda, does contract yellow fever. Mattie does witness its gross effects. Mattie wants to help her mother through the disease; however, Mattie doesn't want to get the disease herself.
In a second important aspect of "honesty," Mattie describes the setting of the day so vividly that the book becomes a perfect example of historical fiction. The setting is Philadelphia (the capital of the United States) in 1793. Mattie's honesty details the particulars of this setting and reveals the activity of an urban environment of the late eighteenth century. Mattie can even see where congress meets and is lucky enough to have her coffee house only a couple of blocks away from Washington's residence! Further, Mattie reveals many important people of the day who visit her coffee house. In an interesting plot twist, Mattie also meets Eliza, a free black person.
Like most blacks in Philadelphia, Eliza was free. She said Philadelphia was the best city for freed slaves or freeborn Africans.
Through Mattie's honesty, the author (Anderson) is able to tackle some of the more controversial subjects of the day that a teenager living in Mattie's time may have noticed.