Catalyzes the transfer of acetyl from acetyl-CoA to desacetylmycothiol (Cys-GlcN-Ins) to form mycothiol. 1D-myo-inositol 2-(L-cysteinylamino)-2-deoxy-alpha-D-glucopyranoside + acetyl-CoA = CoA + H(+) + mycothiol Monomer. Belongs to the acetyltransferase family. MshD subfamily.