While children are encouraged to wear costumes in the parks, adults are not. Unless it's during the Halloween Party, Disney will respectfully request that Grammy change out of her Minnie costume before she enters the park. If she insists on wearing it in the parks, she can be denied admission.
The official "Guest Appearance Guidelines for Walt Disney World Theme Parks" include the following policy:
The Walt Disney World Resort is a heavily themed environment whose world-renowned brand is woven into every element of the show. We reserve the right to exclude from all of our gated attractions Guests whose appearance could create a distraction to the show. Examples that might be considered as such include the following:
Adult costumes or clothing that could be viewed as a costume (unless worn by a child under the age of 10)
Disney, as I'm sure you know, is VERY proprietary when it comes to its characters, particularly the "Fab Five" (Mickey, Minnie, Donald, Goofy, Pluto). What they will be most worried about is that children will see her and assume she's the "real" Minnie, thereby causing confusion for the kids and problems for the characters. And I'm sure Grammy wouldn't want to spoil the magic for other children.
It's a cute idea, but you may want to have Grammy wear her costume to a character breakfast at one of the resorts, and get a photo with Minnie there instead of in the parks. There seems to be a lot more latitude with that sort of thing once you're outside of the actual parks.
