Sadly, this is by no means an isolated incident. For all the pious noise she makes to her children about how the person inside the clothes is more important that whatever he or she happens to be wearing at any particular time, Elly is transfixed by external appearances and doesn't do a very good job of hiding it. Not only is she the judgmental clodhopper she doesn't want her kids knowing she is, she's also terrified that people will look at her children and decide to impale her on a stake for failure to police her devil offspring's wardrobe. She thus sabotages herself by not acknowledging the power of peer pressure. Not only will this come into play a few years ahead when she tries to plead desperately with Lizzie to stay an innocent child and not get the horrible earrings that would have nonexistent people come over and tell Elly that she was incompetent, it's most of why she sided Principal Prurient McAirconditioner in imposing uniforms. It's not about the kids, it's about Elly's need to impress the worthless, the authoritarian and (mostly) the fictitious.
Well, there's that and her need to 'help' her children by reminding them that they aren't old enough to have opinions yet. As an example, the reason that Lizzie was too young to wear earrings and make-up was not that Elly feared for her safety or that she resented the sexualization of the young or any of the politically correct phrases that she parroted. It was that OTHER PEOPLE were trying to hijack Elly's role as sole arbiter of everything ever. It doesn't matter to her that John says unhelpful things about how stressing about what Lizzie or April wear are less important that making sure that they know that Elly has more input that some child who doesn't feed them; what matters is that if some children who Lizzie spends more time with decide things, Elly loses. Thank goodness that Principal Covert Pervert helps Elly out by imposing uniforms and allowing her to yap at April about her wardrobe. Otherwise, she couldn't freak out about nothing and act like a stupid, selfish petty tyrant concerned only about how people think of her.