Why does 'I'm with stupid' have a positive connotation?

I see the phrase ...

I'm with stupid

... used in many occasions, especially on forums using a smiley similar to this one:

It's almost exclusively used with a positive connotation, in the meaning: "I agree with the poster above". But I just don't get why you'd call someone you agree with 'stupid'.

What is the origin and true meaning of this phrase?

It was sometime in the mid 1970's that T-Shirts with the phrase I'm with stupid and an arrow (or actually it was often a hand with the index finger extended) pointing to the left or right (so it points to your companion) were initially available.

I don't think it means anything other than an attempt at wit.

Think of it as a pre-internet meme

This is the closest t-shirt I could find to what the originals looked like.