With Mulan, she appears on little enough "Princess" merchandise that when she does, the only reason why she might be in her "matchmaker" outfit versus the clothing she wore throughout the rest of the film(s) is to fit a continuing theme. For example, the Disney resorts have "Princess" picture frames in which the girls are wearing lovely dresses, with a matching-color picture frame and a snow-globe that holds "their" palace inside. The usuals are Belle, Cinderella, Snow White, and Aurora, but Jasmine and Mulan are also available (Mulan only at the Hong Kong Disneyland afaik). Since all the other characters were in their "ballgown best," they had to come up with something flowing and "pretty" for Mulan and Jasmine, so the artists stuffed them into dress-like equivalents of outfits we saw in the film(s).

In contrast, there's a popular mini dress-up doll set that features Mulan AND Pocahontas, and they only have their "default" outfits (Mulan in her "warrior" garb with her hair up; Pocahontas with her regular dress/miniskirt made of animal hide); they prettied up Mulan by including her pink robe, but it wasn't on her "by default" the way it was for the other Princesses in the set (Cinderella, etc.), and Pocahontas only got a colorful braid-- no weird outfit just to make her more "Princessy."

How do I know all this? I worked for Disney. -_-;