The Games make celebrities of the tributes and fetishize them. That means they have to look their "best," which in Katniss' case means looking like anybody but herself. This was particularly true in the previous book, where she had to look pretty and innocent. In Catching Fire, she has to wear a wedding dress that's not to her taste, for a fake wedding that never even happens. But her stylist, Cinna, understands who she really is and can translate that into a dramatic appearance – bringing some of what's inside to the outside. When Katniss is left to her own devices, she doesn't worry about wearing makeup or looking pretty. She focuses on the things that are really important, like staying alive.