The ideal application is under the mulch. It gets the herbicide right on the soil where it's needed. If you topdress it over the mulch you have to wait of rain to wash it down to the soil. Then the mulch can absorb some of it or keep it off of the soil all together.
If you have to topdress like over existing mulch you should make sure the mulch is broken up and then maybe rake it in a little to help settle it down to the soil. If it doesn't reach the soil it might as well be in the bag.