The mood in The Namesake is nostalgic, yearning, and ponderous. Each of the central characters in the story yearns for something—identity, belonging, home, familiarity, family. This novel is one in which characters constantly look back, either agitated by the past or yearning for it. Ashima yearns to not feel foreign; so does Gogol, though his yearning takes a different direction than his mother's.
This sense of ennui connects directly to alienation. In the Western literary canon, alienation refers not only to an emotion or an experience, but to a dramatic practice. While The Namesake is a novel, not a play, there is a sense in which it provides an example of the theory of playwright Bertolt Brecht, by which audiences are distanced or "alienated" from the characters rather than identifying with them emotionally, allowing them to better observe and understand the characters' context and thereby develop empathy.
In The Namesake, each member of the Ganguli family experiences alienation, made aware of the extent to which their day-to-day decisions embody cultural performance. Ashoke and Ashima "perform" American culture for their children but constantly yearn for a time when performance was not demanded of them—when they could exist organically within the culture they were born to. For Gogol, everything in his life is a performance. He yearns for an organic culture, something he does not have to perform for others. The constant pressures of alienation and performance result in this yearning, nostalgic mood in The Namesake.