The position on Drifloon when you are riding is very poor. You're way over to the side, and not low enough.

Gen 9 and the newest version of pixelmon gave Drifblim the ability to be ridden. I've edited the Drifloon numbers, but I need to see if they're any good.

I tried the numbers for Drifblim and they did not work.

"standing": {
            "x": 0.0,
            "y": -1.3,
            "z": 0.0
          },
          "moving": {
            "x": 0.0,
            "y": -3.2,
            "z": 0.0
          }

I have changed the moving y to -1.3 and we'll see what happens. Presumably, a chance is needed on x, y, or z.