Although popular early on - in 1848 he won a landslide election he frittered this by siezing power and becoming Emperor, this was at a time when France was polerizing beetween growing Conservatism who wanted the Monarchy back, growing Liberalism - who wanted democratic reforms, and growing socialist aspirations of workers who were beginning to organise unions and associations.
Napoleon represented none of these, he was shackled by having no popular base to call on to assert his authority. True he had the army but the French Revolution had proved having the power to order an army about was meaningless, without the backing of a robust social group that could fight and defend its interests on the battlefield of social conflict.
Napoleon I had come to power with widespread acclaim and popular support, this support was robust enough to allow him to institute his Code Napoleon, which was far ranging and far reaching. Napoloeon III could do no such thing, if he came into conflict with any of the social forces vying for dominance in france, he had no social force of his own to call on. He was like Gorbachev in a sense, a bright star without a following, consequently he veers from project to project hoping to achive a victory that will win him acclaim, he was rudderless.
Had he been the King of Prussia or Austrian Emperor he would have had less trouble as they were relatively socially backward compared to France. Thanks to the revolution Liberalism was stronger and had more ambitions in France, France was pregnant with Liberal expectation, but he disappointed them. He was not, and possibly could never have been a Liberal or democrat, he distrusted democratic government believing in the rule of the one wise man.
His mistake was to sieze power and declare himself emperor, he didn't have a robust enough base to cary it off, he had however 3 years earlier won a landslide, a possibility might have been to have championed the liberal and democratic forces, he had no future attempting to champion conservatives or socialists. However this would have been against his instinct and beliefs.
His manouvering on the high wire beween contending social groups, whilst carrying the weight of Napoleon Ist's Mantle portrays a man of unreliable beliefs and aim. I think he did well to last 22 years and have the successes he had.
He was no dullard or fool, he was however undermined by the forces ranged against him and the lack of a reliable natural base of support. I think this shows in his war with Prussia. Had he been more confident and energised - he was also ill -by a strong reliable base he may have led the French army into the Rhineland in a quick preemptive strike causing Prussia to hesitate and falter. He would then have looked very different from the perspective of history.