Why? Don't parrot the party line, analyze your offense.
Did she pass moral judgment on the mentally disabled? No.
Did she say they were less than human? No.
Did she insult them personally? No.
She made an association that acknowledged that being mentally disabled is a sub-optimal status, one that no one desires to hold. Offense at this stems from not
wanting to acknowledge that reality, and guilt at being happy not to hold that status.
Guess what? It's not ideal; get over it. Until the day comes that people are lining up to consume mercury and lead, in order to attain that status, you're going to be stuck with the reality that it's sub-optimal.
Everyone is sub-optimal in some way - we're fat, or unattractive, or not geniuses, or clumsy, or wear glasses, or too short, or whatever. No one's perfect, and jokes about the less common imperfections simply reflect our regret at that, and a sense of consolation that our own imperfections aren't that bad. That awareness is never going away, and sometimes laughing at life's harshness makes it bearable.