I mean, even then, "improving" is entirely subjective. Color grading is the process by which you edit a video to perform global color/tone adjustments, but I don't understand it as being strictly about improving, you might want the video to look old, in which case you'd grade it in a way that it deteriorates.
Really it is not that subjective, there is an art to it, there is some nuance to a mood or style you are going for but there is a definite good and bad.
“Good” or “bad” printing (to use the darkroom photo term) can only really be defined relative to artistic intentions. There are many possible choices to make in producing a final image, many competing aesthetic goals which cannot all be satisfied simultaneously, and no “right” answer.
Some photo printers love to allocate almost all of their available contrast to large-scale shapes, producing essentially silhouettes. Others like to allocate almost all contrast to local fine detail, leaving the image looking like a gray blur from afar but detailed and crisply textured from close up. Some photographers like their images to be a festival of competing intense colors, while others make nearly monochromatic images in one color or another, or stick to a pastel palette, or make mostly neutral images with a few intense exceptions. Etc.
When someone says a photo or video was printed badly, what they usually mean is that either (a) the printer had shallow aesthetic judgment or boring artistic goals, and/or (b) the printer lacked the skill to effect their artistic vision.
