。
She's feeling down right now.
Literal
She [topic-は] now mood [subject-が] is-sinking.
気分が沈む uses the kinetic verb 沈む ('to sink, to go under') for the mood — a perfectly natural metaphor of spirits dropping. The same verb 沈む describes ships sinking, the sun setting (日が沈む), and tone or mood darkening. ~ている here gives the resulting state: 'has fallen and remains in a low mood.' Compare 気分が落ち込む for 'sink into a depressed mood,' and 落ち込む as the standalone verb for being down — Japanese has a rich vocabulary for downward emotional motion.