"She is wearing a skirt."
Translation:هِيَّ لابِسة تَنّورة.
Well, the sentence above is correct but I wouldn't call it a good Arabic.
Your sentence, هي تلبس تنورة is better.
The word لابسة is what we call in Arabic اسم فاعل (ism fá3il) - a noun for the person doing the act or the verb. Such nouns are translated into English in various ways and not in a single specific manner; Sometimes with verbs ending in -ing, sometimes with -er nouns. In a long form, you can imagine this word translates to (who wears, who is wearing). My guess here is that Duolingo contributors picked this word لابسة to match the English translation here (she is wearing). Which, again, is acceptable.
Just to give more examples:
- to walk: مشى (mašá) → ماشي (máší): walking/walker/who walks.
- to hear: سمع (sama3) → سامع (sámi3): hearing/hearer/who hears.
- to drink: شرب (šarab) → شارب (šárib): drinking/drinker/who drinks.
All these examples are done for the masculine nouns. For feminine, we just add ة (-ah) to their ends.