Raccoons could bite and tear through the tarp. They are very strong, dexterous, smart and persistent. Hardware cloth, if firmly attached, will keep them out.
Hardware cloth also allows good ventilation, and the chickens need that. If you seal the coop too much ammonia builds up from their poop and the air becomes poisonous.
I mostly use the 1/2" hardware cloth, I had some 1/4" so I used that in some of the most vulnerable spots.