If you had iFile, you would be able to see that the file is missing from your app package. Because of that, the game has nowhere to pull the file from, so only you cannot see the hat, while everyone else can. Uninstalling and reinstalling seems to be a fix for these types of things in most cases, but if you already tried that on your iPhone, I would try reinstalling it through your computer.

Usually, your device would download the file from the server the second you first see it on your screen. Do you see other people wearing the same hat, or is it only your character that can't seem to wear the hat?