Hi Yimin,
I am not sure whether making staff aware in the store is going to help anything? Because that is what I did.
As I mentioned, I went to the main Apple store on George street in Sydney to check all the new MacBook Air's on display (there was about 4 or 5 of them) and ALL devices had the same issue with the camera, bad quality, pixely and grainy.
When I called a staff member over and compared it to the old model Macbook Air who has the same 720p camera he was shocked and lost for words. All he said was to bring the device back and get my money while it's within the two weeks.
(they are still selling them despite me making them aware of this issue across all Macbook Air devices on display ⚠)
I also called Apple Support on the weekend as suggested above and send them pictures of proof. Likewise they were shocked and transferred me to the highest level (those who work with the engineers). I was asked to do several restarts and updates. Nothing worked.
This was my first Mac and a disappointing experience thus far. I will return the device and go back to Microsoft.