Yes, Wordsworth's title suits the poem well. Both the reaper and the narrator are alone. He, the narrator, appears to be wandering by himself through the Scottish highlands. He speaks only of himself in his fleeting, one-sided encounter with this woman: "I saw her singing ... I listened, motionless and still ..." The poem thus captures a moment of communion the solitary narrator feels with this solitary figure who is not even aware of his presence as she goes about her work.
The narrator likens the reaper to a bird, comparing her to both a nightingale and a cuckoo, creatures of nature singing their solitary songs. Further, her song has a lonesome strain. It is "plaintive" as it fills the air and seems to speak of "sorrow, loss or pain." The lonely cadences of it suit the solitude of the reaper in the empty highlands, so far from the bustle of city life. The solitude is part of the sweet but haunting mood the poem conveys.
It also offers a contrast to another famous Wordsworth poem, "I Wandered Lonely as a Cloud," in which the poet's loneliness is alleviated by the happy dance of thousands of daffodils waving in the breeze. This time he finds a more melancholy companionship as he recalls, as "music in his heart," the reaper's lonesome song.