Dragons and all reptiles have a very simple brain mechanism...
it's called a brain stem.
Reptile brains do not have the outer wrapping also known as the mammalian out core.
Also they do not posses a prefrontal cortex as some mammals do such as the echidna.
See pic:

The echidna has been known to allude scientists and stack bowls to climb out of enclosures etc. They have an enlarged prefrontal cortex which is responsible for abstract thinking and projecting forward and reverse. They are endangered and live in Australia like the BD does.
Most mammals live in the here and now. They do not posses an enlarged prefrontal cortex like humans do. Nor do they have a highly developed neo cortex (thinking mind).
So therefore... Mammals lack imagination and the ability to project forward (future planning) or memory the ability to projector in reverse.
We as humans also posses this high developed brain mechanism unique to other animals... a neo cortex (thinking mind) that covers over the mammalian core.
The human brain is 4 essential components.
#1. The brain stem (ancient mind)
#2. The mammalian core
#3. The neo cortex outer core
#4. The prefrontal cortex (responsible for character traits and imagination)
So to the question of the smartness of the BD... they are not that intelligent. They are instinctual creatures. They understand physical displays with a rigid pecking order combined with aggression. Brain Stem (Reptilian Brain)
They do not have imaginations or really complex thinking abilities.
Although amazingly enough even with such a primitive brain apparatus they do still have unique temperments and their personalities do differ.
So this is what makes them interesting and unique. Also that they are very docile and mild mannered for the most part.
I think some of this comes from selective breeding and the fact that we as humans tend to breed the more affable tempered dragons. Thereby furthering this trait on in its domestic evolution.
The mild mannered docile dragons do seem to do the best in captivity. The easily startled and nasty ones people are more reluctant to work with. And they seem to always be stressed. Which really would be a creature best suited for the wild.
A dragon is not capable of doing tricks and learning complex patterns of abstract behavior.
They will remember certain feeding patterns that are necessary for their survival and general welfare. Self preservation (reptilian brain stem)
On an IQ scale I dont think they would exceed most other reptiles. Alligators crocodiles and sea turtles are some of the smarter reptiles and their ancestors were here with dinosaurs.
Also komodo dragons are almost mammalian and are a creature that scientists see links between reptiles and mammals.
We as humans have a brainstem which is the reptilian part of our brain. This is the ancient part of our minds and then we have the thinking part of the brain... the neo cortex or grey matter. This is where a majority of brain matter exits. (thinking mind)
Check out this site:
http://www.bibliotecapleyades.net/sumer_anunnaki/reptiles/reptiles14.htm