Modern Pokémon titles have made a point to add more character customization so players can express themselves more in-game. Brilliant Diamond and Shining Pearl haven’t dropped that feature, but it does take a while to find yourself some fresh duds to dress up in. Your first taste of higher fashion comes with the introduction to Super Contest Shows, where you have the option to dress for success on the pageant stage.
Otherwise, for a more permanent wardrobe change, you’ll have to wait until after you’ve defeated the second Gym Leader and travel all the way to the east coast of Sinnoh. There, in Veilstone City, you can find a boutique called Metronome Style Shop, which is happy to sell any trainer a handful of outfits to swap out at your leisure. These clothes aren’t cheap, but a few affordable options can improve your style to match your Pokémon Master image.