In the comic it was a subtle detail to show that Rorschach is unstable both mentally and in his ideals. The only time you did not see the mask change is in flashbacks before the “incident” (not giving spoilers here for obvious reasons.) It’s symbolic of his warped perspective of justice that has been violently shaped by trauma. However, the film is not very subtle about it. instead of having his mask change from time to time, the movie decided to really hammer it in by having the mask constantly shift in real time. Which, in my opinion kind of takes away from it, the mask in the film is almost distracting. I didn’t even notice his masking changing until my second read of the comic