Well, if you mean charity as a supernatural virtue, the answer is mortal sin doesn't just weaken it, it destroys it completely. Mortal sin literally causes the death of the soul and is incompatible with the principle of life, or sanctifying grace within it. Charity being a supernatural virtue in the strict sense, it cannot be regained, except by God's own action, through the sacraments.
It is venial sin that affects charity, and it does so indirectly. As mortal sin is opposed to charity, venial sin is opposed to devotion, as St.Francis De Sales treats at length in his classic, "Introduction to the Devout life". So unchecked venial sins weaken our resolves and good intentions.
The Doctor explains,
"But, in fact, all true and living devotion presupposes the love of God;—and indeed it is neither more nor less than a very real love of God, though not always of the same kind; for that Love one while shining on the soul we call grace, which makes us acceptable to His Divine Majesty;—when it strengthens us to do well, it is called Charity;—but when it attains its fullest perfection, in which it not only leads us to do well, but to act carefully, diligently, and promptly, then it is called Devotion.
In short, devotion is simply a spiritual activity and liveliness by means of which Divine Love works in us, and causes us to work briskly and lovingly; and just as charity leads us to a general practice of all God’s Commandments, so devotion leads us to practise them readily and diligently.
So devotion is the real spiritual sweetness which takes away all bitterness from mortifications; and prevents consolations from disagreeing with the soul: it cures the poor of sadness, and the rich of presumption; it keeps the oppressed from feeling desolate, and the prosperous from insolence; it averts sadness from the lonely, and dissipation from social life; it is as warmth in winter and refreshing dew in summer; it knows how to abound and how to suffer want; how to profit alike by honour and contempt; it accepts gladness and sadness with an even mind, and fills men’s hearts with a wondrous sweetness."