Dog intelligence is highly diverse and as individual as the dogs themselves. My kids unanimously conquer that my boy Luke is the biggest doofus we've ever had. Admittedly from a first meeting you may come to the same conclusion as his reactive behavior and puffy blonde exterior can conceal the truth. Luke is probably the most sensitive, aware and attention to detail dog that has ever been a part of our pack.
Many breeds have been labelled the dummies of the dogworld; like the sighthounds or bulldogs. I have completely turned around new guardians way of thinking by showing them how smart a bulldog really is; with simple training techniques. I have only ever met a truly dumb dog once. And that was through no fault of his own; he had actually died on the neutering table and been brought back to life. The vets were not sure if he had lost any brain capacity but I could assure them that he had left most of his brain matter on the other side. Sad but true.
Dog intelligence cannot be summed up in a simple explanation. There are so many forms of intelligence and therefore many ways to display intelligence. Sighthounds are extremely sensitive so trying to train them through the old choke'm methods does not work; they tend to shut down but if you are not experienced in k9 behavior you may see this as "stupid." Take the Australian Cattledog; an amazing dog but one that can be difficult to train because they tend to like to work on their own. Their decision making abilties are extreme.
Personality plays a huge role in how we humans see intelligence as well. When asked to perform a simple task; let's say a Mastiff may sit and look at you like; what? Does this mean that the dog has less intelligence, no. It means that you have not motivated this dog enough for them to consider complying. Some dogs live to do your every beckoning wish; many goldens become obsessed with obedience and your slightest wish is their command.
In my own pack I have seen amazing intelligence. My girl Tilley uses tools to clean her teeth. Luke can get any dog to drop a toy so that he can have it just by barking at the door. And Jessie is the master controller of all and is very much a more instinctive dog than my poodles.
So, there is one blanket statement I will say; there are no stupid dogs.