ISTM that Yookeroo here is using “costume” in the more common sense of “dressing up for a make-believe role that’s different from your real identity”.
There are plenty of kimono-like garments (some of which have actually been called “kimonos” in English for many decades) that I would consider more adaptations of, or homages to, the Japanese kimono than appropriations of it.
Americans creating and wearing something that’s explicitly intended to imitate the traditional Japanese kimono, on the other hand, comes acroos as thoughtlessly appropriating elements of a different culture just for “costume” purposes, to look “exotic”.
A good rule of thumb that I’ve seen for evaluating these situations is the following: “Would somebody from that culture be ‘othered’ by people around you if they were the ones doing that?”
For example, one of the reasons that Black women have complained about non-Black women copying their hairstyles, earrings, etc., is that Black women frequently get told when they wear those styles that they look “unprofessional”, “too ghetto”, etc. If you as a white person can get away with wearing some “edgy” Black-originated style that a Black person would get some disapproval for, then that comes across as treating the style like a costume for its “exotic” look.
But, for example, Black Americans generally don’t get “othered” for eating sweet potato pie which originated among people of West African descent in the American South. That’s something that has percolated into mainstream American culture shared by Black and non-Black Americans alike.
Similarly, a Japanese or Japanese-American woman wearing a plain lacquered hairstick in her updo doesn’t look unusual or “exotic” in most American settings. Hairsticks, even though most of their modern use was originally inspired by traditional Japanese coiffures, are an ordinary item of American hair accessories now.
But a Japanese or Japanese-American woman wearing a traditional kimono outfit on an American street would definitely look “different”, “exotic”, “foreign”, “Japanese”. So non-Japanese people doing that come across as “appropriating” a foreign cultural element for its “differentness”.