Kbin is still a very early beta. We'd be better off helping it grow by suggesting changes to the codebase than packaging them up into an addon.

But no, there's nothing like this that I'm aware of. Nor a mobile app (but the mobile browser stylesheet does a good job, I think).

Edit: Seems I was wrong and there are some userscripts, but I'm glad to see the maintainers are looking at doing some proper PRs into the main branch. We have the chance to help this place grow, and that's awesome.