If the US economy was fine and Americans were happy, why did US invite millions of 3rd World tramps to live including the Mafia after 1880?

Americans were not happy! That was the peak of The Gilded Age where most of the population worked in the unregulated factories we still talk about today. The second largest chunk were poor farmers, many of whom were sharecroppers who had it just as bad as the factory workers. Immigrants were, of course, welcomed because the ruling class used them to fuel their evil machine, where miserable and underpaid workers worked tirelessly to make the rich even richer.