How do you keep vegetables happy? Gather them in an air-conditioned room with plenty of sunlight and toys to play with.
Are you making fun of people that are mentally handicapped?
Relax, I'm just making a joke.
I know you were trying to make a joke, but it's offensive to think that mentally handicapped people can't play an active role in society. There are many inspirational stories of mentally handicapped people achieving great things. Everyone can be a productive member of society if given the right opportunity to shine.
That's all contextualized.
There are very few contexts where that statement would be seen as funny. Most people think that disabled people should be cared for and not made fun of. You should consider the implications of the jokes you tell.