The wonders of magic!
Yeah, I’m just going to assume there is a rather stiff wire(s) running through the golden parts of the outfit making it all form fitting. Either that or she’s rolling the dice that the null doesn’t invalidate whatever magic is holding it up in the back.
That being said, I may be underestimating seamstresses in general. They can be rather magical when they put their minds to it.
It really is a constant miracle that this kimono doesn’t just fall off