Personally, I've always worn dress pants and a "sparkly" top on formal nights. I've never felt underdressed on any of our cruises.
As long as you look nice you'll be fine (especially on Carnival).
"A truly happy person is one who can enjoy the scenery on a detour."
Golden Princess 2009 Carnival Conquest 2010
Grand Princess 2010 Island Princess 2011
Grand Princess 2011 Carnival Magic 2012
Carnival Dream 2012 Island Princess 2013
Carnival Magic 2013 Carnival Legend 2014
Carnival Magic 2015 Coral Princess 2015