Your sautéed zucchini is mushy for one or more of these reasons: 1) You used an old, not-so-fresh zucchini that was soft to the touch. Only get the freshest zucchini for sautéeing. 2) You overcooked your zucchini. 3) You overcrowded the pan with too much zucchini. 4) You cooked it at low heat.