While it may sound like a lot of clothing items, there are well over 300 clothing items at launch to collect. This particular goal may take a few hours and is best done during just regular play.
-Search in every story or building you visit and try on all the clothes. Every item counts whether that's shirts, shoes or glasses.
-You might be able to get this entirely within the mall, that's where I found the majority of my wardrobe as many clothing stores have between 3-7 different items of clothing.
-You can check how many clothing items you have at the full mirrors located everywhere in the world, they're not rare.
I've included a map of Willamette Mall below to help find clothing stores and the full map of Willamette below that. Both are in 4k, so you can click them for a full screen version to look around on. A thanks to Gamerheadquarters for the lovely maps.