YES! denim skirt/jumper will work! Just about anything is a "GO" on casual night - EXCEPT shorts. These casual nights are about as much fun as the "formal" nights. The best part is just being comfortable!
I laugh when I look back at all of the worrying I did about proper attire prior to our first cruise! We are from Ohio - and I gave up wearing dresses when the "pantsuit" thing happened. For casual nights, I wear nice slacks and tops and sandals. For "formal" nights, I have fancier dress slacks and fancy top and I "invested" in 2 cocktail (short) dresses that I drag out for EVERY cruise - never wear fancy things like that around here!