Let us look at this comment that Fred makes in context. After being taken to his nephew's house, Scrooge overhears Fred and his young companion discussing his character, and Fred says:
"He's a comical old fellow," said Scrooge's nephew, "that's the truth: and not so pleasant as he might be. However, his offenses carry their own punishment, and I have nothing to say against him."
What is key to realise is that although Fred recognises that his uncle is "not so pleasant as he might be," at the same time, he also sees, with great wisdom, that Scrooge's character and the way that he reacts around others thanks to his miserly nature brings its own form of punishment in the way that he is unable to participate in Christmas and the joy and happiness of family. The quote also shows the overall goodness of Fred's character, as he is not going hold Scrooge's character against him in spite of how rude and offensive he is. Fred recognises that being miserly brings its own form of punishment without the need for anybody else to punish that individual.