Heat increases the rate at which many fragrance materials evaporate into the air. That is why a car freshie can suddenly smell much stronger on a hot afternoon than it did on a cool morning.
The same effect can also mean fragrance is used up faster during very hot weather. Cars can reach high interior temperatures in direct sun, so follow the product's placement instructions and avoid letting a freshie rest against dashboards, trim, upholstery, or other surfaces unless the maker specifically says that is safe. Stronger in the heat is normal, but proper placement still matters.