Determinants are complicated. One more approach:
stokes[{f1_, f2_, f3_}, {x_, y_, z_}] :=
Sum[{d[y, z], d[z, x], d[x, y]}[[perm[[1]]]]*
D[{f1, f2, f3}[[perm[[3]]]], {x, y, z}[[perm[[2]]]]]*
Signature[perm],
{perm, Permutations[Range[3]]}];
stokes[{y^2, x, z^2}, {x, y, z}]