The daily programme for MSC says evening or cocktail dress for ladies and dinner suit for men on the 2 gala nights they have on a week cruise. That said many do adhere to this, but many also wear less formal evening wear. personally, I love the opportunity to dress up in an evening dress. if you want to be more formal, you won't be overdressed but likewise if you prefer to less formal, you'll find others are to.
Essentially on MSC you please yourself. It's a fantastic cruise line but less formal than others. Black tie is good but lounge suit equally acceptable. Enjoy.