An ugly personality destroys a pretty face meaning. No matter how stunning you may look or how beautiful you may be on the outside if your personality is ugly in some way then this will overshadow your good looks and detract from your beauty. You can be the most gorgeous or handsome person in the world but when you display an ugly personality then no one will want to associate with you and they will see the truth behind the attractive appearance that you present. If you are mean, spiteful, hateful, or negative in other ways then even an attractive face will not be enough to win people over and you will end up alone. It is far better to be average looking and have a good personality, being compassionate and kind to others instead of judging them or putting them down.