So, we get the sense that Mrs. Copperfield is flighty, easily distracted, and spoiled. She means no harm, but she also doesn't have the strength of character to resist a clever, brutal fellow like Mr. Murdstone. Mr. Murdstone flatters Mrs. Copperfield: when David repeats to his mother that Mr. Murdstone called her "bewitching" (2.91) and a "pretty little widow" (2.93), Mrs. Copperfield blushes, laughs, and is generally very pleased. It's these compliments that bring Mrs. Copperfield to accept Mr. Murdstone's proposal of marriage even though her most trusted advisor, Peggotty, can see something is wrong with the man.
Once Mrs. Copperfield is finally married to Mr. Murdstone, the danger of her childish manners really becomes clear. Once they are married, she is totally trapped: there is no way for her to get away from the cruel authority of Mr. Murdstone. Mr. Murdstone immediately starts scolding her, playing a father figure to his new, young wife.
When David comes home from his first visit to Yarmouth, he finds Mr. Murdstone married to his mother and starts to cry. Mrs. Copperfield starts her usual fussing: "Davy, you naughty boy! Peggotty, you savage creature! [...] What a troublesome world this is" (4.10). In response to her tantrum, Mr. Murdstone replies, "Firmness, my dear!" (4.12).