Monster hats could potentially decrease aggro from the type of hat you wear! So, if you equip a monster hat, say the mushroom one, that will make any mushroom-type to not attack you when you get near. This can help players explore without being bothered by monsters around them. This can also lead into an idea for pets! Using the hats can help you tame a monster to fight by your side!
I feel like this will add a sense of social and physical aspect to the game. Just to make Vesteria more unique than it already is!
I hope you like the idea