"Isn't that skirt thin?"
うすい（薄い）is an い-adjective, not a な-adjective.
Here's something to help understand why I've spelt it the way I did. I'll show how the same thing happens with two other い-adjectives that might look a little familiar...
い-adjectives in dictionary form
- はやい（早い）"early, quick"
い-adjectives in adverb/infinitive form
い-adjectives in く → う sound changed form
い-adjectives in a formal ~ございます form
What I wrote was spelt the way I intended. ^^
薄い（うすい）is used for a thin layer of something flat, like for a piece of cloth or paper, or clothing like in this case. 細い（ほそい）means more like "fine line" or "narrow", like would be used for a thread or string. There is also another adjective with the same kanji, 細かい（こまかい）which means something like "finely detailed", like a miniature picture that might be drawn with lines that are 細い (thin)!
Duolingo is getting ridiculous here. Some questions in this lesson ask for the formal form, being くありません and others are asking for the more informal form, being くないです. The annoying part is that they both mean the same thing and yet are not both accepted. Some accept only formal and some accept only informal, with no warning as to which version they want.