As we all know there is no cookie cutter answer. Depends mostly on you as a solo cruiser and what experience you want to have. I can be very outgoing when I choose and find that doing things solo (not just cruising) actually opens me up to more experiences. I can try new things without worrying about what other people want to do. And when you're solo it seems people are more open to engaging with you out of the blue versus when you're with other and they don't want to intrude.
Specifically with NCL, I have been on a smaller ship where no one showed up to any meetings so after day 2 the staff moved on to other duties and I shrugged and amused myself elsewhere, including crashing a honeymoon (couple I saw all the time on and off the ship lol). On the Bliss it was the complete opposite and opportunities for group dinners, shows, cart racing, etc. were arranged daily.
Or you could even cuddle into a lounger on the observation deck and spend your trip reading a book by yourself if you choose.