Said damned thing is a reminder that children are a horrible burden that we all have to endure.
Panel 1: It's morning at the Pattermanse and John is feeling sort of down because of his daily grind. As Elly drinks her coffee and smiles, he SIGHS and says "I don't want to go to work."
Panel 2: As he mulls over the possibility of blowing off the Feggmutzes, Zitzlaffs and Gronvlops so he can chill with the kiddies, Mike yells MA!! from off-panel.
Panel 3: Mike earns his freak-out as he tells Elly that Lizzie
Panel 4: As she rushes off to clean up the mess, Elly stares daggers at John. That's because he says "On the other hand, I hate being late."
Summary: That's because she hates it that he isn't a parent when it's difficult. She knows that he isn't supposed to be because that's how things roll in the Foobiverse but she still resents the laws of "nature" she doesn't really want to change. Not that said resentment is inappropriate, mind you; the real Early John wasn't the kindly goofold of the Later Years. He was a nasty, patronizing, selfish dickweed who treated Elly like a child because she has a uterus so he pulled shit like this all the time. I can just imagine his making a point of not admitting that he's in the wrong here.