I don't understand why did Mikako has choosen a funeral wear for the festival?

:) to forever stay 20? Dunno

im p sure its because her old self died and she became kind of a new person after the entire incident
like when she talked about the new mikako that yuki called out to

It's not a festival, it's a coming of age ceremony. And all along the story, Makiko want to stay 15. The coming of age ceremony is at age 20. By doing it, she's aknowleging the fact that she's ageing, that's a real progress for her, but it's a very painefull aknowlegment. Her youth is dead and she's burring it with the proper Kimono.