"But what if the baker won't let me feel the bread?; you mean to say that after all you are really going to be the kind of woman who the baker won't let near the bread?" (52-53). Figures. Mom has been accusing Girl of being a slut the whole time, so of course it has to come back to that. Mom doesn't explicitly say slut this time, but since she's said it so many times before it would make sense that she means the same thing now. In fact, you could probably interpret "squeeze" (which Mom says earlier) and "feel" as having some pretty sexy connotations.
So in a way, this ending continues Mom's lessons. People like sluts and wharf-rat boys are outside of society. No one interacts with them, so it makes sense that they wouldn’t be allowed to interact with people and do everyday things. One thing that a slut would definitely not be allowed to do is squeeze bread like an upstanding young lady, because her dirty, slutty hands would taint the bread. Obviously.
There is also another possible interpretation of the end. Let's say that Mom and Girl have a bit of a "failure to communicate." Mom talks more at Girl than to her, and she seems to totally ignore Girl. But to Mom, it seems like Girl is totally missing the point. If she follows all of Mom's advice, then obviously the baker will let her squeeze the bread. It's all connected: what seems like a totally random list of advice is actually summed up by this: be the kind of woman who gets to squeeze the bread.
Now, can someone put that on a t-shirt for us?