The locus of plane curves in the moduli stack of curves

Let $d \geq 4$ and let $U_d$ denote the locus of smooth curves in the Hilbert scheme of degree $d$ plane curves. If the members of $U_d$ have genus $g$, let $\mathscr{M}_g$ denote the moduli stack of genus $g$ curves. We show that the natural map $[U_d/\operatorname{PGL}_3] \to \mathscr{M}_g$ is a l