I would disagree - those skills (cover art, typesetting) are orthogonal skills to writing. These are some of the things (I assume) publishers would do for you.

Also, there are many things that I can judge to be good or bad, but that doesn't mean I can produce high quality examples myself. And getting high quality work from others isn't cheap, so it might be out of reach for self-published authors. Even if they know that the cover art isn't good, they might still use it, if they can't do better themselves and can't afford to hire someone who would do better.

That’s a very good point.

It's not about skills. It's about taste. Nothing wrong with getting someone else to do a cover or two. The question is whether the author had even the judgement to ask someone else for better work (in their taste). Same for typesetting. Same for proofreading.

And then there's the french publishers who prefer a text-only cover (and minimal at that) as a matter of their taste (and I guess, expectations of their customers - since not all french publishers go text-only).

Anything is possible, but I don't think judgement of visual aesthetics maps at all to quality of writing.

I'm a damn good technical writer and can break down complex ideas into clear and understandable prose, but I don't know the first thing about fonts or typesetting.

Sure. But only to a point. If it looks laughably bad, then sure it probably doesnt bode well for the quality of the writing. But if the cover is fine/good enough but clearly not something that would come from a trad publishing house... That is when the cover might not be reflective of the work but still hold people, myself often included, back.