Because of the Davy Crockett Indian fighter reference, this would probably be termed offensive to the modern politically correct generation. When we viewed it back when, we didn't think of it as offensive, just funny. But then again, the whole concept would probably be considered unacceptable these days. Making fun of someone who's a little bit slow. How sad that we've come so far in our lack of humor and need to protect so desperately. Perhaps that's one of the problems with modern day society. We're afraid to find anything funny any more. It's not the making fun OF, it's the understanding and acknowledging of the humanity that makes things funny.
In any event, I find this funny. The timing is beautiful, the skill to play badly on an instrument you know so well takes real talent. And deliberately singing off key when you know the difference is incredibly difficult to do.
Also, the relationship and love between the two brothers is what makes their routine so beautiful.