Real medieval knights were trained in the art of fighting and warfare from an early age weren't they. They were the fighting class, it was kinda their job. I dunno how it works in GoT.
Seems to be a title given by lords? I don't remember if it was mentioned how Jorah got his, but we know Stannis knighted Davos so it certainly isn't true that all knights are experts at fighting and direct warfare right? If Tyrion wasn't so disliked by the people he worked for, he would probably have been knighted too after the defense of Blackwater Bay.
So that brings us back to the question - why would Jorah be a better military adviser than Tyrion? Barristan and Daario have shown by their deeds and experience that they would be far better advisers on military matters, but Jorah? Dude is a fucking joke.