Despite being generally seen by historians that I'm reading as being kind of an idiot and overwhelmingly ill-prepared to rule, Charles VIII was apparently at least well-mannered. Frederick Adams Woods describes him thusly at the time of his accession in The Influence of Monarchs: Steps In a New Science of History:

He was then twenty-one years of age, a self-willed prince, deformed in body and weak in mind. He was indeed amiable and gracious, and filled with romantic ambition, but lacked judgment, and his education had been seriously neglected. To ignorance he added debauchery, so there is little to be said in his praise.

A history book from the late 1800s, History of France, from the Conquest of Gaul by Julius Cæsar Continued to the Year 1861, describes him like this:

This young prince was of a gay, lively nature, but so thoughtless and deficient in judgment, that, though he seems to have set out in life with one of the best hearts in the world, he was continually guilty of very unjustifiable actions.

and

He commonly acted from the impulse of the moment, was seldom to be convinced by reason, and had an invincible repugnance to business.

but also

Nonwithstanding these great defects, Charles made himself much beloved. He was generous and forgiving to exess; and had so gentle a temper, that it is recorded of him, that he never, in the course of his life, said a single word which could give pain to any human being.

So based on what I'm reading, it kind of comes down to "he was a really nice, thoughtful guy but a really terrible leader who had no sense of thinking before he acted."