If we tell people that they don’t need to be expert developers to review themes, and that this is a good way to learn better coding practices, why don’t we do the same with design? Good design is just as subjective as code standards (in that the basics aren’t, but people like to argue about it anyway).
The quality of code in the theme repo is improved by the review process, so we should encourage design reviews to increase the quality of design, too.
Some suggestions: fill out the design guidelines in the theme review handbook (maybe with a “how to give feedback” section), then encourage designers to review themes, without worrying about doing the full code review - not all designers are or want to be developers. Publicize that this is something people can do, and it’s still contributing. Encourage people who might not be comfortable with giving design feedback to “self-report” a usability test- write out what they were thinking while activating & setting up a theme.