He was actually a stunningly brilliant strategist in his early career. He had a tendency to be too adventurous, but you could say the same of Patton, Marlborough, and others. Adventurism tends to be a trait of aggressive generals, a side-effect of the same confidence that leads them to thinking outside of the box. As Napoleon got older, however, he seemed to rely more and more upon sheer numbers and frontal attacks, rather than the skilful manoeuvres of his early career. Possibly because his enemies had learnt how to fight off his favourite strategy - that of simply bypassing any armies stronger than his own and only attacking smaller, weaker detachments who were unprepared for battle - by this point, but more likely simply because he had become very, very arrogant in his success.
There really is no reason why Napoleon couldn't have pulled out a diplomatic solution in 1814, except that he genuinely believed himself to be so good that he could beat back all of his enemies, except maybe Russia, and buy time to prepare for yet another war with Russia. He said so himself. Funnily enough, once France's position was absolutely hopeless - after the Battle of Nations (Battle of Leipzig) - Napoleon, low on troops and with no remaining allies (Denmark doesn't count when she has no troops to send you), returned to the short, sharp manoeuvres of his early career, and successfully fought a rear-guard action far longer than most men could have, while attempting to negotiate for his son to take the throne.
I'd also like to say that claiming Napoleon was a bad or even mediocre general because he didn't mind the words of Master Sun is just laughable. Sunzi was a military philosopher, and while his treatise is decent enough as an introduction to military philosophy for teenagers, it's terrible as a practical manual for warfare. I believe it was Euripides who said; "No battle plan survives contact with the enemy." That concept, which even junior officers will agree with, was alien to Sunzi's writings.
Sunzi's book is more of an attempt to apply early proto-Daoist thought to warfare. It seems decent enough in theory, but it breaks down the second you attempt it in practice. You can even recognise that upon reading the Chinese commentaries appended to the book itself, since many of China's greatest generals, while publically praising master Sun, seem to blatantly ignore his instructions on a regular basis (Cao Cao, Mao Zedong and Chiang Kai Shek immediately spring to mind as people that operated to a more, shall we say, Caesarian military philosophy than Sunzi's, and all three of them have written commentaries on The Art of War, with Cao Cao actually being the most prolific commentator on Master Sun to this day).
