What is an Ugly Christmas Sweater? According to fashion experts it is any sweater considered tacky, gaudy and flat out in bad taste. So how did these Ugly Christmas Sweaters become the latest rage, well you do not have to look any further then the reason you would want to wear one in the first place, and that is for an Ugly Sweater Christmas party. This latest themed Christmas party has caught on internationally. The trick behind having the ugliest Xmas sweater according to one source is to have as much decorations and color as you can.