let’s start with the basics, no race in WoW that has reached sapience thought and is capable of learning is dumb on a genetic level, some might have issues speaking outside their native tongue, but not a single race is just pure stupid.
Now for Ogres in particular, they basically had an apolcalypse and lost most of their knowledge, they didn’t keep records on pen and paper nearly as much as they used the magical technology created by the arakkoa for record keeping, the Apexis Crystals, they also have different moral standards then traditionally are considered ok in this (Modern) day and age, which makes them seem overly barbaric. Ogres can even build complex machinery, as seen when kargath uses the weapons built into the Arena of Highmaul in his fight against the players, as well as being the first and foremost practitioners of geomancy, the first dreadnoughts were created by Ogres from enchanted stone.
Why are they so dumb on azeroth? Well it’s likely several reasons, Education is a big one, any remnants of the Gorian Empire on other parts of Dawgar (the gorian name of draenor) would have a very difficult time getting information to azeroth, and most of the 2 headed ogre magi aren’t natural or made by Gorian Sorcerers, but are Mass Produced version made by the shadow council so they could help in the Second War, meaning very few Ogre Scholars are on Azeroth. This combined with the fact many of their leaders where killed by the old horde has forced them into a more scattered, tribal society.