ΔfGΘ vs T plot in the Ellingham diagram slopes downward for the reaction

C(s)+12O2(g)⟶CO(g);ΔS increases.Hence, as the temperature increases, TΔS increases and hence ∆G (∆H -TΔS) decreases. In other words, the slope of the curve for formation of CO decreases. However, for all other oxides, it increases.