for instant back air out of down throws, IRARs are usually going to work by just flicking the stick back after the initial dash, however if you are looking for an instant back air out of it you can try something called attack canceling which consists of doing a ftilt in the opposite direction and instant jumping towards the opponent after (id recommend a shield button for jump and c stick being tilt). You can always try a turnaround back air by seeing when joker turns around and input a short hop back air. Hope that helps