Is it permissible to decorate the bride's car with flowers on the wedding day, knowing that this is an old custom and tradition in the questioner's country?

There is no harm in decorating the bride's car with flowers on her wedding day, for the basic principle is permissibility, and it is something that brings joy to the bride and groom and their families. However, extravagance should be avoided, based on the Almighty's saying: (And eat and drink, but be not excessive. Indeed, He likes not those who commit excess.)