While Carnival has relaxed its dress code to allow them into formal night, on the last few cruises I have been on, most people still dress up. You will never be OVER dressed though so if you want to dress up, do so. You could get away with capris if you didn't want to where a dress but like Sonny said.. they take lots of elegant night pictures which we use as our family portraits so I say dress up.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013