Yes, it is understandably offensive - because autism is involentary - but is there really a lot we can say to change it? Not to sound horrible ou anything... but, when toi think about it... that's part of Seth MacFarlane's point of Family Guy's humor: to poke a finger a society, and not specifically at one group of people. Just most people in general.
Plus: they've stepped out of line so many times... and they've a dit much worse in other episodes... so why whine about one 'joke'? They'll keep on écriture these kind of things as long as the montrer goes on for. Get over it.