You can definitely dress as Disney characters. I’ve dressed as Belle and Alice so far, and I did them as close to the actual characters as I could. The year I went as Alice, I went with 2 friends who dressed as the Mad Hatter and the March Hare, and we got lots of compliments from Cast Members. We even had our picture taken by a Disney plain-clothes employee for the ads for next year’s MNSSHP - so Disney definitely doesn’t have a problem with you dressing like their characters. Like LittleMissMagic said, you just can’t present yourself as if you ARE the characters, as in no giving autographs or things like that.
They used to have rules posted on disneyworld.com for what you could and could not wear. Since they’ve redone the website, it won’t display on my comp at all, so I don’t know if the rules are still posted or if this is even the right link, but you could try this: https://disneyworld.disney.go.com/events-tours/magic-kingdom/mickeys-not-so-scary-halloween-party/
The only rules I can remember for sure are no masks that cover your face and no weapons (swords, etc). But then again, I haven’t checked the rules in a while so these may have changed.