He's the worst military commander ever.
In the Battle of the Bastards he charged head on because he got overly emotional, playing right in to Ramsay's hands and getting his army slaughtered. In this one he allowed the dothraki cavalry (a large bulk of their forces?) to be annihilated in about 10 seconds because they charged with zero support, and then spent the rest of the episode trying (and failing) to be a hero.
He doesn't command, he's just a great warrior. People keep talking about him as a King or a leader when really he's just very good with a sword and a honourable bloke. He's led two battles and his army has been destroyed in both, and he's only 'won' because Sansa and Arya came up with the clutch.
The Dothraki should have been forced to fight on foot and clearly instructed that cavalry charges will not be effective against this kind of enemy, Jon should have known that. That would have been 40 thousand more men (some of the finest warriors they have to boot) holding the line rather than thrown away at the start.
I hope someone calls him out for all of it. Doubt it though.