By anyone's standards, Napoleon in Animal Farm is a very bad leader indeed. If a good leader is someone with a sense of mission higher than themselves, someone capable of galvanizing others together in pursuit of a common purpose, then this lazy, despotic pig most certainly doesn't fit the bill.
Napoleon may pay lip-service to the principles of Animalism, but that's about it. Whatever else may be said about it, Animalism is, at face value, a noble creed that seeks to build a fair and equal society in which everyone pulls together for the common good.
But the very idea is anathema to the incorrigibly selfish Napoleon, who, unlike Old Major, sees the Animalist revolution as an opportunity to grab whatever he can get his greedy trotters on, whether it's political power, a comfy bed, or Mr. Jones's alcohol.
As Napoleon's only in it for himself, he's unable to lead the other animals in a common endeavor. All he can do is terrorize them to get what he wants. But Napoleon's capacity for terror and violence doesn't make him a strong ruler; quite the opposite, in fact. Indeed, one could argue that Napoleon's despotic rule is a convenient way of covering up his numerous shortcomings as a leader.
When it comes to the crunch, in those moments when leadership really matters, Napoleon's missing in action. Take the Battle of Cowshed, for instance. Here, the animals were engaged in an epic fight to defend the Animalist revolution from the hated human oppressor.
But Napoleon was nowhere to be seen. If there was a hero that day, it was Snowball, Napoleon's bitter rival. No wonder that once Napoleon's sends Snowball packing from the farm, he cynically rewrites history to make it seem as if he were the hero of the hour.
As well as being lazy and selfish, then, he's also a proven coward, someone who runs away when the going gets tough. These characteristics combine to make Napoleon a bad leader.