I don't have a Mavic Air but do have a Phantom 3 Standard and a Spark.
In ideal lighting conditions the auto setting will give you pretty good results. That being said, I usually always use manual exposure and shoot 99% of my photos at ISO 100. The more you increase the ISO the more noise you introduce into your photo (more grainy) and this becomes more apparent in lower light. I always have the histogram enabled (a graph that shows the tonal values of your image). I then adjust the shutter speed so that the graph (usually looks like a mountain) is pretty much in the centre. This usually gives me the best exposure possible. Keep in mind that the sharpness of your image is affected by other factors as well such as wind and shutter speed. The gimbal does a great job stabilizating the camera but its not perfect so any camera motion will result in a less sharp image. Also, keep in mind the limitations of the camera. It's no DSLR and is basically an action cam mounted on a drone.
As Fly Dawg said, 100mbps has nothing to do with still image quality - its the video bit rate and affects video recording.
The best way to improve your photos is to practice - try out the different camera settings and see what the results are. Learning how to use the histogram really helps with getting the best exposure possible with the equipment and lighting conditions at the time of image capture.
Hope this helps.
Chris