The person who said this isn't too bright either, by look of it, or at least he/she doesn't have a very good grasp of grammar. As I'm sure you know, it should of course be:
"You're a whole hamper short of a picnic" ( not 'your')
This is an extension of the more common insult 'You're a few sandwiches short of a picnic' , which suggests that the person has a few brain cells missing.
In this case, the speaker has taken the insult a few steps further. The picnic not only has a few sandwiches missing, but an entire hamper (or picnic basket). Or in other words, he/she has no brain at all.