I'd rather have been right; that's because we're reminded that Lizzie is trying to destroy Elly's brain.
Panel 1: We start off with Elly leaning against the door of the washroom; as she holds onto the knob, she says "Do you need help in there, Lizzie? Call Mommy if you need help."
Panel 2: A second or two later, she opens the door and says "Okay, that's long enough; Mommy's coming in to give you some...."
Panel 3: Elly freezes off-panel as she surveys the damage; we start off with a smiling, naked Lizzie holding a toothbrush in a poorly-drawn toilet. There's toilet paper and cotton balls strewn all over the floor, a potted plant and what I assume is a shampoo bottle have been knocked over and there are handprints on the wall. All told, it would take a normal human being about ten, maybe fifteen minutes to straighten all that away.
Panel 4: Since Elly isn't a normal human being, she yells "HELP!!!!" at the top of her lungs; we can also assume that she'll sit there catatonic for an hour or so.
Summary: That's because she can't simply frown, shake her head in vague discomfort, clean up the mess and get on with her day; she's got to spend hours moaning about injustice. It might seem to an outsider like John that she could avoided that by supervising Lizzie like she's supposed to but that's not a "fair" thing to say to a busy mother who has no help and no time for herself.