Catalysis of the reaction: malonyl-CoA + an anthocyanidin 3-O-beta-D-glucoside = CoA + an anthocyanidin 3-O-(6-O-malonyl-beta-D-glucoside).
malonyl-coenzymeA:anthocyanidin-3-O-beta-D-glucoside 6\'\'-O-malonyltransferase activity