That earlier strip was also predicated on the idea that Elly had a few screws loose. As I said when I described it, she seemed to have a tunnel vision of the mind's eye that made her forget both Lizzie and the hot-ass curling iron the minute she stopped paying attention to them. Making a joke of Elly being so dumb that she couldn't foresee the outcome "Lizzie will grab this thing if I give her half a chance so I'd better keep it from happening" doesn't make me much happier than the more modern "Elly is too dumb to understand that someone's just messing with her" because it isn't any more humorous than "Elly turns into a shrieking moron at Lizzie because she did something Mommy was too stupid to allow for."
The question that is left is why is it funny and cute that Elly is angry all the time and as dumb as a sack of hair? Surely Lynn could have arranged things so that Elly doesn't have to look like a short-tempered imbecile all the time.