This wasn't exactly just a Dothraki v Lannister fight. Drogon broke their line, then he obliterated it. Also the Dothraki has the element of surprise. Sure the Lannisters got into a defensive line but that's not even remotely the same thing as having a battle plan for the Dothraki.
Yeah, you are right about Drogon. I should have mentioned it but I don't think Dothraki will be useless as I thought. Tehy would have showed something. Instead, they were badasses who could stand on a horse, shoot arrows then jump to another horse.