The typical stoner stereotype is not a favourable one. Lazy, low-functioning and unkempt are just a few adjectives used to describe your every day pothead. But is there any evidence to suggest that regular consumption of marijuana causes the user's brain to function at a lower capacity than it would without cannabis?
Short term memory is certainly impacted after consuming cannabis. This manifests itself in the form of forgetfulness. You might be having a conversation with someone and lose track of what you were talking about, or leave the room to get something and then forget what it was you wanted to get. Certainly not stupidity, but it can be frustrating for the consumer.
There is currently more research available on cocaine (in regards to animal testing) than there is for cannabis. The long term effects of consuming cannabis are not clear right now, but hopefully in the future research will reveal how marijuana and the chemicals within it affect the human brain.