Translation:The skirt is not bad, but really short!
Probably yes. But notice that "ganz gut" is less good then "gut". It's rather like "acceptable, but I don't really like it"....
The same is true for "ganz schön", "ganz nett", "ganz ok".
As I explained in the other comment, it means something like relatively beautiful, applied to a noun. So don't use it for German women or their clothes if you talk to them.
Linguee shows plenty of results for the adverbial ganz schön, that is used to modify an adjective or a verb and means pretty, quite or pretty well. But here we have only ganz as an adverb modifying the adjective schön. This doesn't occur on lingue.
Have a look at https://de.m.wiktionary.org/wiki/ganz there you can also see how it this works. What I'm referring to is called a Gradpartikel (graduation particle), that alters the degree of something. Combined with adjectives like gut, schön, nett etc. it lowers their degree, making them only relatively good, beautiful or nice.
I also know how they're used, because I am German ;-).
Danke. I was really confused here. I thought ganz was "entirely" and was very confused why "The skirt is entirely good, but completely short" was wrong. It didn't sound quite right in English though. So the Duolingo translation was very confusing. Thanks for clarifying.
Unlike "not bad" in English, which can mean "really impressive" or "so-so" depending the inflection of the voice, here "ganz" is really used as a diminution of "gut".