The key point in your question is "look". "look" is so-called linking verb. It is like a Be-verb which would follow with a subject compliment (主詞補語). This subject compliment is usually an adjective.
Both worried (past participle,過去分詞) and worrying (gerund,動名詞) can function as adjectives. Now, the question is which one to use. In your case, both are possible and correct, because it doesn't have an additional statement to tell people why she worries or why she is worried.
Let me give you an example:
A worries B
A is worrying.
B is worried.
If the sentence is more comprehensive as:
She looks quite worried, when she is told that she fails the test.
She looks quite worrying without knowing the result of the test.
Then you can determine which to use is better.
Some people would say "worrying" is only to describe things. Not really, for example:
He is a worrying person = His personality is worrying.
Here are a list of LINKING VERBS:
Be-verb, look sound, smell, feel, taste, appear, become, get, turn, stay, grow ...
To determine, whether a verb is a linking verb in the sentence, all you need to do to replace that verb with "Be-verb", if the meaning of the sentence doesn't change dramatically, then it is a linking verb. For examples:
It tastes awful. ("taste" is a linking verb)
(if you replace "tastes" with "is", the meaning is about the same)
He tastes both cakes. ("taste" is not a linking verb)
(if you replace "tastes" with "is", the meaning changes).