Would not bother if I was you - it is so hot in the med at the moment and we have just got back and some folk did dress smartly in the evening but did not see anyone in djs and I am sure that I read somewhere that you can now wear jeans into most the dining rooms on NCL.
Leave the DJS more room for your clothes and shoes!!!
I am sure that you will have a great time.
We will be going there this coming weekend. We cruised with NCL this time last year and we only saw a few gents wearing DJs (on the day the formal photos were taken and on the day when some guests dined with the captain). If you want to dress smart for a special occasion, bring a suit and ties an some shirts. My husband did this way last year on our anniverary and my bday. A few more people were dressing this way too.