There is nothing that can be added to the Mosquito Barrier to reduce the smell as that garlic smell is what repels insects from the area in the first place. The smell of the garlic juice will become unnoticeable to humans within a few minutes and will only be strong during and right after application.
Mosquito Barrier is meant to be applied to lawns, ornamentals, edibles and trees, not hard surfaces like wood. It should be applied to the underside of leaves of bushes, shrubs, ornamentals, and low lying tree branches as that is where the adults lay during the day and it will help to repel them from areas it is applied to which can help around the porch if close enough. Otherwise something like the Dynatrap Indoor / Outdoor Insect Trap with Optional Wall Mount (DT1100) could be used up on the deck.
Mosquito Barrier is a completely natural garlic based spray that can be used on any crop safely and will not affect humans or animals.