Walt Disney dressed well, as you are reminded by his statue when you enter the Magic Kingdom. He wants you and your family to follow suit (pun intended).
Here's the official Disney rule: "Proper attire, including shoes and shirts, must be worn at all times. Costumes may not be worn by Guests ages 14 or older. Clothing that drags the ground is prohibited. Clothing with multiple layers is subject to search. We reserve the right to deny admission to or remove any person wearing attire that we consider inappropriate or attire that could detract from the experience of other Guests. Visible tattoos that could be considered inappropriate, such as those containing objectionable language or designs, are not permitted."
