I’ve briefly mentioned it before, but my theory is that that part was only thrown in because the showrunners thought girls liked stuff that had to do with fashion, which I feel is only backed up with the more recent episodes focusing on Marinette’s designer career being related to Adrien in some way.
I also neglected to mention this when talking about it (and I really should have gone more into detail regarding my final thoughts), but Heartcatch Precure has a much better subplot revolving around fashion that goes into the idea of why people are so invested in it.
At the beginning of the series, Erika/Cure Marine starts a Fashion Club at her school with Tsubomi/Cure Blossom and some other recurring characters. At first, it seems like an excuse for Tsubomi and Erika to spend more time together, but things change once Itsuki/the future Cure Sunshine joins. Because she intentionally acts masculine in order to inherit her family’s dojo, Itsuki has a real desire to dress in more feminine clothes, a desire that gets explored as she gets more focus.
Several episodes focusing on the Fashion Club tend to explore the philosophy of fashion design, viewing it as a form of expression. When Itsuki’s brother sees her wearing a dress she designed herself, he comments that it’s what the “real” Itsuki looks like. Once she becomes Cure Sunshine, she realizes she can still kick ass while dressing in overly girly clothing, and starts wearing more clothing like that. When we see Itsuki start to wear the female school uniform after the final battle, it shows how her mindset on fashion and expressing herself has changed.
Then over the summer, another story arc related to a fashion show put on at the school fair is set up, with Erika choosing the theme “A New Me”, reflecting all the personal growth the main characters have gone through in the series. The idea of connecting fashion to personal expression feels consistent, and really helps the audience connect with these characters and their interest in the subject.
Miraculous Ladybug... really doesn’t do that for any of the characters with dreams. Not even the episode centered around dreams and goals. Most of the episodes where Marinette designs something is connected to Adrien in some way or another, and we still don’t know what exactly got her into fashion.