You know, after thinking about this for a moment, this is really sweet for a non-obvious reason. Screw Ball isn't saying that she thinks Flutters is her mom. She's not calling objects random things, she is calling them by names that make a certain kind of sense, just in a roundabout way. A book isn't LITERALLY a window, but it's a saying that it's a window to another world. So she's not saying Flutters is LITERALLY her mom, she's saying that, if you apply a roundabout sort of logic, she's like a mom. She's caring, and nurturing, and kind.