Do they still get cutie marks? Are they treated with tolerance? How about Ancient Ponies? I had this idea, one comedic and the other serious about two uninformed Pegasi officers of Ancient Equestria attempting to draft a mentally challenged Pegasus.

Dramatic
Officer Stormy Weather frowned and grabbed the stallion who was cringing, had been cringing since they'd stepped on his family's cloud field. "You are a Pegasus and your country needs you! Buck up and-"
A long wailing cry broke from the stallion and he started flapping his wings, but not to fly. An older stallion stormed out the door. "What in Celestia's name are you doing to my son?!"
The stallion raced to his father, hiding behind an outsretched wing.
Cloudy Weather raised an eyebrow looked to his companion. "Let's go."
"But-"
"Stormy, I said let's go."

Comedic.
Stormy Weather nodded. "I realize your son's mentally challenged, but that's no impediment to being a full member of Equestrian society. Why he could even become an officer or senator."
Cloudy slapped his forehead. "Do you listen to yourself?"
"My son can't be a senator, he lacks the capacity to lie."

6761369 They just put them in the mental facility.

6761377
Barking like a dog is a symptom of a ruff life.
#poniesworkinghardlikeadog
#hoofandpawpatrol

6761369
"My son can't be a senator, he lacks the capacity to lie."
Yep, gotta reroll that char again. Need at least a +5 to Prevarication skills.
+10 for to be a successful liar. + over 9000 to be an American president.

What does mentally challenged mean in Equestria?
What would it look like and act?
Do they have IQ tests?

Would using human references work?

Officers may need to lie too.