You are right, QR codes are very easy to decode if you have them raw, even the C64 should do it in a few seconds, maybe a minute for one of those 22 giant ones. The hard part is image processing when decoding a camera picture - and that can be done on the C64 too if it has enough time and some external memory (or disks for virtual memory). People have even emulated a 32-bit RISC processor on the poor thing, and made it boot Linux.
In real mirror pics, the phone is always perfectly aligned with the frame (obviously).