I think it's good we don't have shadows...
Personally when stoned I find myself afraid of my shadow, then again the cats totally wig me out too, I'm convinced they know.
As far as why people think stoners are always stupid. One word, marketing.
Stupid somehow became funny, Cheech and Chong capitalized on this. Later Sean Penn played Spiccoli and made an ass out of himself and people who thought they didn't know any stoners just assumed that the media charecterizations were accurate.
Today, stupid is still funny, but people are starting to realize it's BS as more and more 'professionals' come out of the smoking closet. I can't say what I'm responsible for professionally, but my daily responsibilities include providing digital security to over 6 million people, I can't be 'stupid' and do my job.
Also 'stupid' without weed has become 'in comedy.' See Adam Sandler and Will Farrel. They are categorically idiotic without the assistance of any herbal involvement.
Another thing I know that when I'm stoned I find some of the stupidest stuff funny as hell, when sober I find myself thinking wow that really wasn't all that funny. That might have something to do with it and general higher tolerance for stupidity when stoned.
-RT76