Angry is a more accurate term, I agree. But, in everyday language (esp. in N. America), we use "mad" as a synonym of "angry" very frequently (if not more frequently). Here are some examples:
She is mad at me because I did not say hi to her. She is mad at her boyfriend because he looked at another girl. My mother was mad at me for saying ....... . The woman is mad at me for because I said she's ugly. Why are you mad at me?!
In all of the above examples, you can substitute "angry" for "mad" and the meaning will be exactly the same.
Then, you might use "mad" in a diff. context. Eg. Are you mad?! Which would be more like "Are you crazy?!".
So, there you go. It all depends on the context, and since there is no context, my answer ("She is mad") was correct, and it's also accepted by Duolingo.
In Polish "zły" can mean bad, wrong, evil or angry. "Mean" is "złośliwy". There's no such a construction like "being bad at something" - we don't have a word for that.
You can say "on robi to źle" (he does it badly) if you mean that someone is bad at doing (!) something, etc.
If you want to say that someone is bad at something, you should use something similar, just like "On nie umie (can not, doesn't know how to do...)/nie rozumie (doesn't understand) matematyki (maths)." If you have any doubts, notify me.
But there is an adverb słabo, which apart from its literal meaning weakly/poorly, is also used in few set phrases:
słabo się czuć – 1) (z czegoś) to feel weak (at something) 2) to feel sick/nauseated/faint
słabo (komuś) się robi – (somebody) is getting sick/nauseated/faint
"Is there?" Well, sure there is, every adjective and every noun has a form for every case.
It just won't work here: you won't say "Ona jest złą" on its own. Noun phrases can take Instrumental in such phrases (Ona jest złą kobietą), but a standalone adjective just takes Nominative.
Look, context is important to meaning and it generally doesn't translate from one language to another. While "niedobra" has the contextual meaning of "bad/naughty", it's way more appropriate for everyday use which is what this app is supposed to be teaching. If you prefer, you could also use "złą osobą" which is "evil person" but "zła" in this context, without the person part, means angry.