Only those whose entire understanding of the world consists of Christians like ma and “Dangerous, Evil Other” would think that. It’s a Wonderful Life “is a fitting metaphor for a nation absent Christian belief.”
The obvious falsity of that can be seen right now, as Benton Harbor Michigan becomes a real-life “Pottersville” under the Dictatorial hand of the Emergency Financial Manager appointed by right-wing Republican Governor Rick Snyder to run the town. Please listen to the video linked below, and as you do, bear in mind that Rick Snyder, Representative Al Pscholka who wrote the Emergency Financial Manager law and who will profit from it, and Whirlpool Heir and owner of the new Golf Development, Fred Upton are all right-wing Republicans who claim to be models of Christian morality.
Meanwhile there are towns and nations run by people from religions of all different stripes and by those who largely do not subscribe to any religion who are acting more like George Bailey, the Jimmy Stewart character in the film. In other words, while the religious right is largely like Potter or empower those who are, many atheists are the antitheses of him.