It's not a BAD portrait - just not the very best. (Albert Finney in the musical is another very good one, and Michael Caine in the Muppet version is a tremendous performance wasted in an awful movie.) When George C.'s being Nasty Scrooge, you can see a little twinkle back of the nastiness, as in "see how nasty I can be!" He chews the scenery a little.
Sim has noodled out a believable psychological transformation for Scrooge. Scrooge knows that he's mean and nasty, and he knows that it's wrong, he regrets his wasted life, but he's so weary and bone-tired and old that he thinks it's too late for him to change. His tired old eyes when he looks up at the two solicitors for charity are heartbreaking. And when he is transformed by the realization that it is Christmas Day and not too late after all, it's splendid and hilarious at the same time.
And when Mervyn Johns as Bob Cratchit tries to bear up and be cheerful but finally breaks down and whispers, "Oh, Tim! My little, little Tim!" it makes me cry just to think about it.