WHY do you not like some code? To avoid pointless discussions about taste and gut feelings, I'd bring it down to falsifiable statements.
"This is hard to maintain, for reasons X and Y" is such a statement. As is: "This is hard to test for reason: It needlessly engages in combinatorial explosion" is another.
Yes, you could shorten that to 'ugly'. Good for you.
Nevertheless, I've seen tons of discussions referring to 'ugly' code where the reasons were solid but completely different from maintainability (for example: "It is longer than needed", "It does is not idiomatic for this language"), or even not particularly solid ("It is ugly because it isn't functional", "It is ugly because the style guide says all APIs should always be interfaces", "It is ugly because I see tabs, not spaces").
> Or it could look “clean” but have a bad Bug-O.
I think this is possible but in practice rare. I'd be curious to see an example of code you think falls into this bucket.
But this is precisely what people mean when they say it's ugly.
"Ugly" is what this gets translated as by your limbic system which registers in an instant: "fuck this big mess and the work it's going to be to reason about."
Aesthetics and usability are deeply entangled here, and the problem with that code is absolutely an aesthetic one. I want to say, more so. Because I experience it that way first, and only later do I enumerate the measurable reasons it's likely to be more buggy because of it.