Leaf mulch is not the same thing as topsoil. But most lawns can use the addition of organic matter such as the mulch will provide. Just be sure you add a thin layer, no more than a half inch or so, or you might smother the grass. If you add it after you core aerate, some will trickle down into the holes and be very beneficial. Be sure to pick out any large pieces or trash since most municipal mulch is not terribly fine and often has non-organic bits in it.

Thanks for your response. It is helpful.