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Is that Mary the life of the party, or what? She goes through life boozing and busting things, and when she isn't perpetrating violence against her own children, she's badmouthing them to anyone who will listen.
Arguably, there are two kinds of drunks: those who feel terrible about themselves and their actions the next day, and those who just get drunk again. Mary is of the latter breed. She's so busy drunkenly condemning everyone else's behavior, in fact, that she barely notices that everyone—street urchins included—is making fun of her. As far as Mary's concerned, she is awesome, while everyone else is lacking.
Like everyone else in the book (except Maggie), Mary is on a serious high horse. Along these lines, she makes a huge show of her outrage even though she has only ever displayed the most despicable behavior herself.
Not sure what we're talking about? She kicks her daughter out of the house for being with Pete, despite this pretty much dooming Maggie to a life of danger and even worse poverty. Mary is all about spectacle, so when she does this, she "pac[es] to and fro, addressing the doorful of eyes, expounding like a glib showman at a museum. Her voice rang through the building." Because of this flair for the dramatic, when Mary forgives Maggie in the end, we get the sense that she's doing so for effect—not because she has any kindness in her heart.