。
She doesn't care in the slightest about her test score.
Literal
She [topic-は] test [genitive-の] score [about-について] not-even-a-little [is-not-minding-気にしてない].
Three pieces stack to deliver the indifference. ~について ('about, concerning') is the standard formal way to introduce a topic of attention — slightly elevated compared to に関して or simple about-talking patterns. 少しも＋negative ('not at all, not in the slightest') is the universal degree-negation frame: 少しも ('even a little') paired with a negative predicate denies the slightest amount. 気にする ('to mind, to worry about, to take to heart') in its negative ~ていない (here contracted to ~てない) describes a sustained state of unconcern. Together: the speaker is observing complete, durable indifference.