The Mall is where you spend diamonds on cosmetics. It's organized into a few sections: profile pictures, avatar skins (grouped further into sub-categories you can tab between), banners (also called album pictures, used to decorate your profile), and consumables like XP boosters.
The Mall also has a rotating Featured section highlighting a changing set of picks — it's worth checking back every couple of days, since what's featured changes over time and might surface something you'd otherwise miss.