The reason that this is important is that it's just another example of something I've noticed about Elly and, to a lesser extent, John. I've noticed that she seems to be almost outraged by the fact that they want to wear clothes that are fashionable instead of practical. Whenever she puts her foot down about a trend she doesn't like, she makes a lot of noise about how she thought that she could delay the confrontation a few years. What distinguishes Elly from other parents who are actually concerned that children are growing up too fast is not a fear for Lizzie's well-being; what separates Elly from the people chatting about cameras in houses is that she hates the idea of Lizzie growing up at all. Remember, this is the same woman who screamed in agony because the child turned two and was no longer recognizable as the cooing lump she was 'meant' to be. We see this when she freaks out because Lizzie wants earrings, when she wants to wear makeup and when she wants to get contacts. Simply put, the idea that Liz might grow up before she's ready to handle it horrifies her almost as much as the idea of spending time with her.
Another factor that must be considered is that Elly hates the fashion industry in the first place for its on-going failure to provide her with clothes that can silence her self-hatred. Since she expects them to do what she will not and make her satisfied with what she sees in the mirror, it's sort of a losing proposition for Liz and April to mention the word "style" in Elly's presence.
We must also remember that Elly is obsessed with the fear that the world is a non-stop orgy and cannot decide if she should envy it or fear it. The same woman who clearly lusted for all the juicy details of what she falsely assumed to be the NC-17 rated movie that was Connie's lonely life is also horrified by the prospect of having sex herself as the intimacy and release of tension frighten her no end. Any reminders of said ickiness must therefore be denied.
This makes John being a dick who wants to play fashion cop because he doesn't want to look bad in front of the neighbors seem healthy, normal and good in comparison. As long as the children pay fealty to him, do nothing to embarrass him and remember their place, he doesn't really care that much about the petty crap that transfixes Elly.