In the inventory, where I have multiple pairs of Traditional Mickey Ears, instead of stacking as one item and showing the number, each pair has its own box. It's kind of cluttery.
No, I bought two extra pairs of ears last night and they didn't stack. Maybe it started after the green flip hat was retired? Or it's just entirely random and some people's will stack and others won't? It could definitely be annoying if you have a bunch. It's too bad we don't have a closet sort of item we could store clothes in so they aren't cluttering our inventories.