。
She's muttering to herself.
Literal
She [topic-は] alone muttering is-saying.
ぶつぶつ言う ('to mutter, grumble') pairs the mimetic ぶつぶつ with the verb 言う to capture the half-audible, repetitive grumble that characterizes complaining or absent-minded talking-to-oneself. ぶつぶつ is a 擬態語 (state mimetic) that depicts the manner of speech rather than its content, and the same word also describes a row of small bumps or pimples on skin — the underlying sense is 'small, repeated, scattered units.' ~ている here gives the ongoing-action reading: she is in the middle of muttering, observed by the speaker.