Resort causal attire is always appropriate aboard NCL ships. Slacks, skirts, and collared shirts are all you need.
About 30% of the passengers will dress up on Lobster night. That includes shirts with just ties, sport jackets, business suits, up to some tuxedos for men. Nice dresses, pants suits, coctail dresses, up to some formal evening gowns for women.
The rest of the passengers, that's 70%, are still dressed in resort casual on Lobster night.
It's Freestyle Cruising, and it's your choice.