In the US, that is an unthinkably bad swearword for some reason.

That's heavily dependent on regional/cultural factors. Among a younger and (mostly) gayer demographic, the once-feared "C-word" is very commonly used, especially in its adjective form.

How do you use it as an adjective? The bad thing was always labelling someone with the word, but there isn't really any other way to use it.

We Aussies have many ways to use it as a compliment

Sick cunt, Mad cunt, Good cunt Etc

It does feature several times.