There will be Christmas decorations, the singers/dancers will do a Christmas show (probably). With that itinerary they might do it Christmas day when you leave Cabo. (On our Christmas cruise they did a show on Christmas Eve but that was a sea day.) You might want to check into what will be open on Christmas day in Cabo - may not be much shopping available. Christmas is not "over the top" on the ship so I don't think you will get sick of anything.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014