There are a number of different hats available for Flo to wear.  Hats can be found in a bunch of different places: some hats can be won in Mystery Boxes or special Events, and some hats can be purchased directly in the store.  

Each hat looks different, and some hats will give Flo a buff that will help in the game.  You can see all of the hats you own (and all of the hats available in the game) if you go into any diner store and click on the hat tab.  If you click the "Equip" button under a hat, Flo will put that hat on.  Flo can only wear one hat at a time, and she will be pictured in the diner with the hat she is currently wearing.



Once you own a hat, it is available for Flo to wear in every venue, and, while some hats are purely decorative, some hats will give Flo a buff that will help in the game.  You can see what buffs (if any) are associated with a hat by clicking on the hat in the store tab.