When a key portion of that happiness is announcing the occasion (whether or not it is actually the occasion) to literally everybody who sees you because of the big colorful button, then feeling special is a key part of that "something."
It's not just that someone had a birthday in the relatively distant past. It's not just that they somebody knows that a celebratory feeling was somehow deferred in order to enjoy that feeling at WDW. It's that someone wants to walk around wearing a colorful button, or tiara, or just married Mickey ears or whatever.
The indicator means you want to show off. The indicator means you want everyone who sees you to know that today is your birthday +/- three months. The indicator means you want people to notice you and understand that today you stand out from the crowd because unlike everyone else you have had and/or will have a birthday. And by golly, if people feel moved by community spirit or corporate policy to congratulate you, why that would be a nice little bonus. But obviously not expected. You don't walk around wearing a button that says IT'S MY BIRTHDAY when it's not even your birthday because you want people to say happy birthday! That would be ridiculous! #NotAllBirthdayButtonWearers
Wear whatever you want. The OP asked if it was frowned on, and I'm over here frowning, and I'm not the only one. But it doesn't matter to me because the proliferation of randomly delayed "birthdays" means I can follow a simple policy: I will never say happy birthday to someone wearing a birthday pin because I am pretty damn certain it is not that person's birthday.