@Mikasa2077 Traditions are passed through generations, but they can also be fairly recent. For example, the tradition of a bride wearing all white in the West only started around 200 years ago. Even if we assume ancient China had the red dress tradition, that isn't mentioned in this chapter. I read it again and it's clear the emperor isn't mocking her. He is analyzing her style and giving constructive advice.
@MayhemTrinkets says this too. Also, this is not the first time I see a story which claims that certain colors are appropriate for certain body types. It's obvious this chapter is just one more example. You could say like
@MayhemTrinkets that the author intended to have a double-meaning, but the in-context words of the emperor character are definitely referring to the girl's literal height.