I think it's fun to say "HITLER WAS A STUPID DUMB DUMBY LOL" but that's not the truth. We just don't want to admit that sometimes the most intelligent charismatic people are capable of being despicable evil monsters.
I wouldn't exactly say he was a genius though, he had the intelligence to organize a political campaign, write speeches and play the political game to a certain degree, most of his success though was due to luck, there were many people like him trying many different political strategies for personal gain it's just that his style turned out to be the most succesful. Men like Goebbels who were more politically intelligent but less lucky would rise through the nazi ranks but Hitler had already taken his place as head.
Of course Hitler made several blunders in military strategy, he held back the blitz when it was about to capture the British and French army at Dunkirk, he diverted the luftwaffe to bomb London instead of the RAF, he didn't put as much effort into Operation Sealion as he did into Operation Barbarossa, he antagonised the balkans unnecessarily wasting and months postponing operation barborssa, he failed to properly equip operation Barbarossa as a contingency in case things went wrong, he fixated on capturing cities with millions of troops in gruelling urban warfare when he could have laid siege to them with just a few divisions who totally outclassed the besieged if they were foolish enough to try and break out, he failed to commit to north Africa, he failed to properly support Italy with instructors, industrial support and technology. His entire grand plan may have been wrong, many in the west and eastern europe would have leaped at the idea of allowing Germany and Japan to invade the Soviet Union in the wake of Stalin's purges, Germany had a population of 50 million and was far more developed while the Soviet Union had 100 million and was pretty much a third world country so it was entirely feasible if he didn't have to contend with the west as well. Then, after developing his military and taking control over most of Russia it wouldn't have been a challenge to brow beat eastern europe and blitz France. The US would have probably stayed isolationist at that point.