If you don't know, there is currently a petition started by a couple with a daughter with Down Syndrome to see a Disney Princess with a mental condition. The reason is so people with mental conditions have more positive role models (because lets face it, the media is not kind to people with any kind of mental health issue) and at the times of this writing, the petition has over 50,000 signatures.
There is however people who don't want to see such a princess or don't trust Disney. Some don't want to see character with mental conditions because they don't want to think about there own or feel they won't work in a cartoon. Others feel that Disney will make an offensive stereotype or a character with a mental condition would be easily mocked by the audience.
For me, it all comes down to the way it is treated. I have autism (i was diagnosed when I was 2 and re-diagnosed and assessed when I was 10) and honestly sick of Autistic people in fiction treated like they are anti-social cowards, or victims or unintentional villains. Only 1 out of every 1000 autistic person is a savant but yet that is the only version we ever see is that and even then I makes it out they're bad people or act all the same. I'm sick of every fictional character with it being, "the autistic person" instead of , "a person who happens to be autistic".
If Disney was to make an Autistic character, I think someone similar to like Teen Titans Starfire or Twilight Sparkle, a shy or hopeful character who is also strong willed and head strong would be in my mind be a great role model for Autistic children and not at all mockable.
Admittedly I can't think how you would deal with a character who has down syndrome or other conditions but I'm not the best person to make that judgement and it doesn't mean it can't be done.