Davis would have preferred to be Commander of the Confederacy's armies rather than President. So given his choice Davis would be in a position more like Halleck than Lee.
His record as an officer in the Mexican war seems to have been decent enough. He doesn't seem to have shied from leading his regiment in combat at any rate. And he called in a favor from President Polk to make sure it was equipped with the latest rifles.
All in all, I don't think his political career tells us very much about how good Davis would have been in field command. All of the good generals in the Civil War went through a learning period after some sort of defeat and came back better generals. That in mind I think Davis in command would be a decent officer in the field with the potential to be a good one if he learns the right lessons.
EDIT: I forgot to mention, Jefferson Davis would have been a hell of a lot better than some of the generals the Confederacy had IOTL