The poem is a short defense of someone who has been labeled "angry." The title, "Angry Bird," sets up that label, and the first line pushes back at once: "She's not angry." The speaker insists that what looks like anger is really sadness and hurt, a pain people mistake for aggression because it shows on the surface.
The speaker seems protective of her. The last lines turn toward a "you" who judges without knowing: "You don't know anything / About her life, / So you don't understand." There's no shouting here, only plain, flat statements, which makes the speaker sound tired of having to explain. The poem doesn't tell us what has happened to her, and that gap matters. Her story stays private, and the poem suggests nobody has earned the right to name her feelings without knowing it.
The mood is quiet, tender, and a little defiant. It asks the reader to look past a quick judgment and think about what might be underneath.