I would not put a tarp on the bottom -- its better to have hardware cloth that keeps out predators and allows mooisture to drain. Tarps around the sides will be good in the cold months to keep out the wind and rain/snow. A tarp over the top now will be good to provide shade and keep out rain!
Lots of duck keepers use tarps! I'm kind of fussy and want the duck coop to look smart as well as be functional. However, most people are more concerned with the function -- protecting ducks from inclement weather. I have seen plenty of blue tarps tied across structures that are duck houses!!
