One Who is Not Loved
Ah, okay.
I just remember when people were really offended by her, like the writers/animators were purposely making her seem retarded.
And it was the fans that were complaining about it.
That's pretty surprising...
She's meant to be ditzy, hence the name Ditzy Doo, but she was never portrayed as retarded imo. She's just called Derpy because of the DERP meme, which features people with their eyes crossed or going in opposite directions.
The episode with her got some flak because some people thought Derpy was an insult, I think, and also because the female voice actor doing her lines thought she was a male, so the voice sounded a bit weird. I personally saw a lot more backlash when they changed it, though, than I heard about anyone complaining in the first place. Either way, she's still pretty popular.