In the story, Peggotty is angry with David's mother because she thinks that the latter is making some very poor choices.
In chapter 2, Peggotty tries to reason with Mrs. Copperfield. She tells Mrs. Copperfield that the man she is seeing will not make a fit husband for her. For her part, Mrs. Copperfield thinks that Peggotty merely wants to take control of her life.
On the contrary, Peggotty can clearly see from experience that Mr. Murdstone will be a cruel husband. Essentially, the faithful servant tries to warn her mistress, but she will have none of it.
Peggotty also begs Mrs. Copperfield to think of David and how things may change for him if she marries Mr. Murdstone. For her part, David's mother remains unconvinced that she needs to stay away from her admirer. She rationalizes her behavior and argues that Mr. Murdstone has yet to discuss marriage. However, Peggotty tries to warn Mrs. Copperfield that marriage is Mr. Murdstone's goal.
As for David, he takes an immediate dislike to Mr. Murdstone when he meets him. Later, we discover that Peggotty's instincts have been right all along. Mr. Murdstone does marry Mrs. Copperfield, and he does make the lives of his new little family miserable.