The rating system is quite limited and clunky compared to systems found in the wild. The concepts under the hood are all sounds this is purely a superficial ux problem. For instance in the moodle.org forums the UX looks like this:

But in the wild ratings are much better represented as thumbs up / down (eg youtube) 

or emoji (facebook)

Or 1-5 stars:

Allowing a rating to be configured visually is better in a bunch of ways:

1) its one click instead of 2

2) its instantly clear what it means and is easier to scan

3) its more familiar with user accustomed to these common design patterns

So proposing either a 'simple' or a 'complex' solution to this.

The simple solution is where the UX is fully managed by core and would be a big improvement while not being super flexible. There would be a new file area against each set of ratings which would be almost identical to the one used in  assignfeedback_editpdf for stamps:

/admin/settings.php?section=assignfeedback_editpdf

Each file would map 1-1 with the name of the rating. When a rating apis detects that images are present it swaps out the UX from a select dropdown to a list of images and you just click on the image you want. This would handle the youtube thumbs up / thumbs down use case quite well and the facebook emoji to some degree but would not handle a star rating.

For more complex solutions the theme could override of the simple one but allow it to be completely replaced by the theme so that advanced things are possible such as hover animations (facebook emoji) or a different widget (eg the 1-5 stars all combined).

It might be useful to include one or more of these most common ratings as shipped scales bundled with a relevant renderer in the core themes so this all works out of the box.