Plot3D[x[t] + y[t] + m[x[t], y[t]], {t, 0, [Pi]/2}, PlotRange → All,
PlotPoints → 80]
It wont plot it just copies it
It’s most probably not a Plot3D thing, but something for Plot, because Plot3D wants two parameters, but you give only one: t.
looks like you’re trying to do a parametric plot.