I would have to agree on the comedic aspect of this movie. I was in the 5th grade and everyone in the class must have watched the movie the night before. Our teacher came in that morning and heard everyone saying "WAH-JAW," and a long lecture followed. The fact that something so horrible on a basic level was so widely viewed is certainly a testament to Shaun Cassidy's popularity at that time.
The humor is juvenile and inappropriate. I know that. But, just this past month, we got a new employee at work named Roger, and I swear I heard Linda Purl saing "Wah-jah" in my head. Think of the time frame of the movie, and then think of Shaun Cassidy's target audience. Young teens and tweens in the late 70s. That's why we all thought it was funny, we were kids. It had a lot more to do with Cassidy's and Purl's performances than the subject of retardation. BUT, attitudes toward the disabled were quite different in 1979. "We didn't know better" might sound lame, but we really didn't. And that immature kid still lives inside us, or at least it does me. Though, I now have the good sense to know that even if it strikes my inner child as funny, it's inappropriate.
I am a disabled person and I find parts of this movie funny and do not feel it is inappropriate to laugh at it. My and my sister's imitating the speech of Roger and Virginia was not out of cruelty, but just like you would find anything funny that is also sweet and cute. I did learn to never call any mentally challenged person or anyone else retarded. It is a word I block on all my social sites, too.