Should Catholics in a state of grace call themselves sinners?

Yes, because they sin if say they are without sin.

Council of Trent session 6 on Justification, Chapter IX. "Against the vain confidence of heretics" (Denzinger 802):

But, although it is necessary to believe that sins neither are remitted, nor ever were remitted save gratuitously by the mercy of God for Christ's sake; yet is it not to be said, that sins are forgiven, or have been forgiven, to any one who boasts of his confidence and certainty of the remission of his sins, and rests on that alone; seeing that it may exist, yea does in our day exist, amongst heretics and schismatics; and with great vehemence is this vain confidence, and one alien from all godliness, preached up in opposition to the Catholic Church. But neither is this to be asserted—that they who are truly justified must needs, without any doubting whatever, settle within themselves that they are justified, and that no one is absolved from sins and justified, but he that believes for certain that he is absolved and justified; and that absolution and justification are effected by this faith alone: as though whoso has not this belief, doubts of the promises of God, and of the efficacy of the death and resurrection of Christ. For even as no pious person ought to doubt of the mercy of God, of the merit of Christ, and of the virtue and efficacy of the sacraments, even so each one, when he regards himself, and his own weakness and indisposition, may have fear and apprehension touching his own grace; seeing that no one can know with a certainty of faith, which can not be subject to error, that he has obtained the grace of God.

Trent also defined these de fide dogmas:

CANON XIII.—If any one saith, that it is necessary for every one, for the obtaining the remission of sins, that he believe for certain, and without any wavering arising from his own infirmity and indisposition, that his sins are forgiven him: let him be anathema.

CANON XVI.—If any one saith, that he will for certain, of an absolute and infallible certainty, have that great gift of perseverance unto the end,—unless he have learned this by special revelation: let him be anathema.

It's perfectly fine to call yourself a sinner, as in a fallen human being prone to sin, or having sinned in the past, even if forgiven. It's a statement of humility, and with such there is nothing wrong—its even virtuous in certain cases. However, it can become a "cloak for malice." We should live, says, St. Peter:

1 Peter 2:16 (DRB) As free, and not as making liberty a cloak for malice, but as the servants of God.

Just because we have been forgiven, and recieved grace, and escaped the wrath of God, it gives us absolutely no right to sin in the future. For:

Sirach 5:5-9 (DRB)  Be not without fear about sin forgiven, and add not sin upon sin: 6 And say not: The mercy of the Lord is great, he will have mercy on the multitude of my sins. 7 For mercy and wrath quickly come from him, and his wrath looketh upon sinners. 8 Delay not to be converted to the Lord, and defer it not from day to day. 9 For his wrath shall come on a sudden, and in the time of vengeance he will destroy thee.

TL;DR Inasmuch as it is right to call someone who has sinned a sinner, it is perfectly fine to call oneself—or even in certain contexts, another—a sinner. Inasmuch as the sins forgiven one who is justified are no longer existent, but blotted out, it would be wrong to call oneself 'sinful' or 'in a state of sin,' because this would be false, and an affront to the grace of Christ, which 'remembers your sins no more' (Hebrews 8:12).

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

Being in a state of grace is defined as:

Condition of a person who is free from mortal sin and pleasing to God. It is the state of being in God's friendship and the necessary condition of the soul at death in order to attain heaven.

However, the Catechism makes a distinction between sins as mortal or venial so it is possible to be in a state of grace (free from mortal sin) while still being a sinner (having committed venial sins). Indeed, the Catechism refers to one who has committed venial sin(s) but not mortal sins as a "sinner" despite being in a state of grace (emphasis added):

1863 Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul's progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not set us in direct opposition to the will and friendship of God; it does not break the covenant with God. With God's grace it is humanly reparable. "Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness."

One could perhaps claim to be not a sinner just after having received absolution (so that you are free from mortal and venial sin), but even so the Catechism uses the word "sinner" to refer to a person who has just received absolution (emphasis added):

Satisfaction

1459 Many sins wrong our neighbor. One must do what is possible in order to repair the harm (e.g., return stolen goods, restore the reputation of someone slandered, pay compensation for injuries). Simple justice requires as much. But sin also injures and weakens the sinner himself, as well as his relationships with God and neighbor. Absolution takes away sin, but it does not remedy all the disorders sin has caused. Raised up from sin, the sinner must still recover his full spiritual health by doing something more to make amends for the sin: he must "make satisfaction for" or "expiate" his sins. This satisfaction is also called "penance."

The Catechism could easily refer to the "sinner" who "must still recover his full spiritual health" as, for example, a "penitent", yet it nonetheless continues to refer to the person as a "sinner".

Therefore, I would still call myself a sinner even if I'm in a state of grace. Even if one has recently received absolution from confession, in honesty and humility it is probably not long before one commits another venial sin and can no longer claim to be free of all sin.