Shooting with a 600mm lens on a crop body is tricky under certain conditions. In terms of IQ a lot depends on the conditions. IQ can go down the drain if you shoot at a longer distance AND over any sort of surface that radiates heat back into the air, e.g. asphalt on a sunny day.
This is nothing knew we all know that. What's more remarkable though how much it can affect image quality and how much it changes by the second. The other day I was testing my new 150-600 outside. I know it's a pretty good copy, shots indoor are phenomenal. But of course we shoot wildlife outdoors 
Anyway, I tested that lens plus my Sigma 120-300 2.8 OS + 2x TC. Most targets I shot were so badly affected by the conditions that both lenses seemingly sucked big time, and it wasn't the lenses' fault.
I did an experiment by using a tripod, live view, manual focus and a remote trigger. I aimed the lens (Tamron) at a fence post, only about 40 feet away. It was a cold day with very bright sun that was heating up most solid surfaces.
I shot 10 photos at f/8 at 600mm manual focus through live view. I did not touch the camera/lens, I only snapped photos with the remote. There were pictures only 1 second apart where one was sharp, while the next one was unacceptably blurry.
When you shoot the Moon, you're aiming up, often close to 90 degrees. That makes a huge difference. There are still some heat currents impacting the image (the Earth radiating heat out to space) but it's much less severe than shooting something near the ground on a sunny day.
So basically the bottom line is that it's tricky to test long lenses under certain conditions. You may take a couple photos with your lens and think that it sucks, or your camera sucks. In fact it might just be the conditions that moment. A photo taken the next moment might be much better.
Here is my Moon photo from the other night. When I shoot the Moon I use live view, manual focus, manual mode, remote trigger (or timer), and I take a lot of photos since even the Moon is affected by the quickly changing heat currents. You can even see that on live view when using 10x magnification. So that's why it is important to take at least 5-10 photos to find a nice sharp one.
70D, using a Sigma 1.4x TC:

IMAGE LINK: https://flic.kr/p/qj1PSd
Moon Tamron 150-600mm 1.4x TC 70D
by
gabebalazs
, on Flickr