Well, to the question " Why do people think deformed individuals are mentally challenged?", I would answer this:
To find the answer, as always (or often if you like it more), you must seek it in the past.
When in the olden days a child was born deformed or with a handicap, it was naturally discarded from society, and left to die, because it was different from them, a "spawn of the devil". If one survived, it would become the scapegoat of the whole community, wether it be the children or the adults, because he was ygly, because his legs were bent, because his back was hunched. If something happens, its his fault because he is different and acts strangely.
But why does he act strangely? Because he was not educated or "nurtured" in a human society, nobody taught him right from wrong, what came from appearances, etc, etc. Because of this, people thought the individual stupid,because to them these things were evident, but he didn't know as no one told him. Thus deformed=stupid is born.
As an example for that, look at
The Hunckback of Notre Dame
But that was then you might say, today we are tolerant, today we understand these people and integrate them in society.
In what universe is that?
Today, modern medicine and global hygiene and wellfare have reduced considerably the number of deformed children being born. Thus they are rarer, and that rarity accentuates their differences.
Of course, their family will look past this most of the time, or prentend to at the least. But still, we, as a society, see these people as pseudo-aliens to us.
Our ego and our pride tell us that these "things"can not be part of the human race, because that would put us on their level.
Thus we "beastify" them, we correlate their minds with their shapes, refusing to acknoledge them as our equals. That is why, in my opinion, this reaction takes place. Simply because we can not accept for them to be different from us.
I can't say I am impartial on this, because I too have fallen into my egos trap. But
this is something that must change for the better in our society, and that means accepting ourselves as imperfect, which since the middle ages hasn't been so successful.
Cheerio