From what I've read and seen it should be OK. Carnival even sells special day decorations in their Bon Voyage site. Then they will decorate the door, the room, etc before you all arrive. So, I don't see why you couldn't decorate yourself too. If your balloons are too big then you'll just have fun popping them.
Go for it and have a good time decorating. I doubt they will allow the baloons if they are big and stick out too much but people do decorate doors and it's yours to decorate. It will also be sort of a marker for other cruisers who have rooms near yours. When they are in the halls looking for their rooms, they will look down the hall and see it and think, 'OK we're almost home.' Have FUN!!!
MY SHIP HAS SAILED
Carnival Conquest 10/26/03