I have symbol that contains a profile photo that is masked into a circle shape. And to create a list of them, I would use the Override Image feature. However, if the image is not of the same aspect ratio, then it will stretch. Is there a way to retain 100% width throughout all images?
No, Sketch (51.2) doesn't provide option to retain 100% width for overriding images in symbols. As Billy said in comments, to avoid squished images you should crop your profile pictures to same aspect ratio.
If you are fine with random profile pictures in your prototype, you can use Tiny faces sketch plugin to generate random profile pictures.