More the electropositivity of central atom, the more basic is the oxide and more electronegative central atom, more acidic is the oxide.
As we know, the electronegativity is the property of the element to attract electrons pairs. Hence, acidic character of the oxides "decreases down the group" because the electronegativity decreases down the group. Electronegativity decreases down the group because the "distance between the nucleus and valences shell electron increases. Hence, acidic nature of oxides increases left to right and decreases from top to bottom.