Die Hard isnt a Christmas movie

I can't entirely agree with your reading-- aside from another message of the film i.e. the value of a man and how everyone makes a difference in the lives of those around them, sometimes in unexpected ways.

However, the anti-capitalist message absolutely is present. Mr. Potter is depicted as a fat cat concerned only with money. When George sees the alternate reality where he never existed, Potter has transformed the town from small town Americana to a seedy and callous neon strip. He puts profit over the lives of others, no matter the consequences, and is representative of unfettered Capitalist greed. When George Bailey's business is saved at the end, it is because the common people (stand-ins for the working class) come together to save his loan company. Socialism saves the day from the uncaring capitalist.