They can wear whatever they hell they want, I don't really care about character outfits. I never understood the obsession with changing up clothes every game or every couple of games.
As for Mickey, the new outfit is Noctis' casual clothes, including hat and winter jacket.
Good question, I have no idea as to why people like this either. I am totally one of those fans, however; I really like when characters change outfits as well. It's probably just some type of superficial appreciation to the art medium at hand, I don't think there's any real reason they have to have new clothes in each main series title, though. They just look cool I guess?