A “perfect” creature is the one that has the ability to function imperfectly.
To the contrary, I will say that the ability to function imperfectly is an imperfection, or at least implies a state of imperfection. Further, when a rational creature functions or chooses what is objectively wrong, it adds to his imperfection, as when an evil choice creates an evil habit or disposition.
This is why I say that we are better than stars, for example. They can only function according to the laws they are given (like the one of gravity).
Absolutely speaking, we are better than stars, are worth more than stars because we are composed of, in addition to matter which we share with the stars, an immortal, spiritual soul which can know and will.
However, relatively speaking, anything in nature (stars, trees, flowers, deer) that are what God wants them to be, according to their natures, as God created them, are better than man when he chooses something that is contrary to his nature, and thus is not existing according to God’s will.
We have the possibility to defy the laws God gave us (we can commit adultery) thus we are better, if we agree that a creature that has the ability to work imperfectly (and perfectly) is better than one that can only work perfectly.
We have the power to defy the laws of God for our nature, but it is not because we can misuse that power that makes us perfect, it is because we have the power of rational choice in itself, in the first place, that makes us ontologically superior to non-rational creatures.
However, the potential to misuse rational choice indicates that the will of man, in this life, exists in a state of imperfection. When the will is perfected, then it is wholly incapable of choosing evil. The will is perfected in the vision of the Divine Essence, in which it can only choose God, it cannot turn away; and this is the condition of perfection and absolute freedom of the will.
By way of an analogy, we know that God is perfect and lacks no perfection of His nature. And God cannot choose evil. For instance, God cannot lie. God cannot do evil because He is perfect being. That God absolutely cannot choose evil in no way represents a limitation on His infinite power. It belongs to the perfection of absolute Goodness not to be capable of evil, even potentially able, to choose what is contrary to His nature as infinite Goodness.
In contrast, since man is a finite creature existing in this life in a state of imperfection he can choose evil. When man is perfected in Heaven, his will is confirmed immovably in the Good.
I think it is the vice of pride, which makes us think that just the ability to choose wrong is real perfection, power, freedom and dignity. Adam and Eve fell for that line. If they would only choose such and such they would be as gods. As a result, they lost their freedom and perfections given by preter-natural graces.
One of the effects of Original Sin is that we do not will the good that we ought, nor are we able to do the good that we will. The will has become enslaved. Choosing evil makes the will a slave to sin. Only the grace of Christ can free the will to choose rightly. That is why we have the Sacraments. And if we choose rightly, we will be perfected in Heaven where we can only choose God.