Why is the statue of the Happy Prince a figure of charity?

The Happy Prince is a figure of charity because after a life of pleasure behind palace walls, he realizes (after death, now that he is a statue in the town square) how much suffering there is in his society. He sees hunger and need as he never did before, and his compassionate heart wants to alleviate the problems of the poor.
He is a figure of sacrifice because he encourages the Swallow to strip off and sell the gold and jewels from his statue self. He wants that money used to help the needy, not simply sitting on him as an idle showpiece. He doesn't care if selling these costly parts of himself make him look shabby or ugly if he can help people.
As the story states:

Leaf after leaf of the fine gold the Swallow picked off, till the Happy Prince looked quite dull and grey. Leaf after leaf of the fine gold he brought to the poor, and the children’s faces grew rosier, and they laughed and played games in the street.

The Happy Prince is willing to look bad if he can bring joy to children.