Should Catholics in a state of grace call themselves sinners?
The short answer is yes.
Being in the state of grace does not mean we are not sinners, but that we still share in the presence of God within their souls while being yet sinners!
The state of grace does not mean that we are without sin or without any guilt due to former personal sins committed against one of the ten commandments given to Moses on Mount Sinai.
Although we may be in the state of grace, we are nevertheless not sinless like the Virgin Mary though a divine privilege which God accorded to her alone.
It warms the heart, to say the rosary in French, because they add the words, poor sinners in their Ave Maria.
Je vous salue, Marie pleine de grâces; le Seigneur est avec vous. Vous êtes bénie entre toutes les femmes Et Jésus, le fruit de vos entrailles, est béni. Sainte Marie, Mère de Dieu, priez pour nous pauvres pécheurs, maintenant et à l'heure de notre mort. Amen.
Please do not forget the words of St. John the Apostle:
If we say that we have no sin, we deceive ourselves, and the truth is not in us. - 1 John 1:8