I haven't heard of any mods that would allow you to put a hat on your pig friend sorry. If you were willing to put in the work yourself, your best bet would be a resource pack that uses optifines CEM feature to change the pigs model to have a hat, and then a name tag on your desired pig to change its texture to be hatted :). You would have to get optiforge working for optifine to work alongside your mods, but its a client-side thing, so you dont have to worry about the server.
