From the WDW website:
Attire that is not appropriate for the parks—and which may result in refusal of admittance—includes but is not limited to:
- Adult costumes or clothing that can be viewed as representative of an actual Disney character
- Masks
- Clothing with objectionable material, including obscene language or graphics
- Excessively torn clothing
- Clothing which, by nature, exposes excessive portions of the skin that may be viewed as inappropriate for a family environment
- Objectionable tattoos
Adults can visit the parks if their hair (or makeup, if applicable) resembles a Disney Princess or character, provided that they are not also wearing a costume or clothed to look like the character.