When I start the mavic up, the camera is slightly tilted even when it is on the ground. How do I fix this?

I think it's C2 (right trigger) + camera dial (right) wheel... I don't have it handy, but try that.

OK, I lied, it was handy.  That is the correct instruction.

Please try to calibrate the gimbal and IMU. You can calibrate the gimbal at gimbal settings.
Here is the tutorial for IMU calibrating:

Aaand as usual, Thor offers better instruction...