Leading a brigade of cavalry, or even a division, Van Dorn was a decent General, with his attribute capable of being mostly a positive influence at those levels of command. At the head of an Army he was an imbecile, being way too impulsive, acting on intelligence without confirming it, or even taking into account his supply, with his habits of blaming his own mistakes on others and his apparent "chasing skirts" also speaking badly of his character, but highly suggesting he was naturally such a recklessly impulsive man. His complete abandonment of Arkansas, was not only a foolish military decision that no one prepared for that level of command would do, much less as eagerly as he did. Plus he did it after just being asked to support Beauregard, and he up and abandoned his post with his Army and left an entire State to the enemy, for an attempt at reinforcing Beauregard, which he never did, acting independently, committing an unforgivable action in a military situation.
Van Dorn was a glory hound who didn't like bad raps, and let the chip on his shoulder lead him into taking absurd levels of risk in almost every aspect of his life, (the title "terror of ugly husbands" he is attributed speaks for this as well, especially since he was married), heck in a lot of ways he reminds me of Custer, but the sad part is Custer was probably a more capable officer, and was apparently more honest. I'd say he was recklessly impulsive, in the performance of his duties, and his lifestyle. In his lifetime his service, and death was best summarized by an Atlanta newspaper:
"If he had led a virtuous life, he would not have died unwept, unhonored, and unsung. Think of the universal respect paid to the lamented Jackson. The whole country is filled with mourning and tears at his death, while no man expresses even a regret at the fate of Van Dorn."
Newspapers ain't the best of historical accounts, but a Confederate newspaper in Atlanta speaking that way of a murdered Confederate General speaks volumes.