It's hard to say with how preseason works.
In the past he's been overshadowed by many other champions. His ultimate while good doesn't really protect you at all when facing enemy teams grouped up. Other assassins rely on high mobility and untargetability to get to their targets and escape. With Nocturne all you have is a spellshield. That's not good enough protect yourself. Now there is ways to make his spell shield very hard to break by getting more spellshield items. Mainly edge of night. Don't buy banshee's veil, the ability power doesn't help you at all. I guess Nocturne's advantage is he can build defensive items and use them decently unlike a Zed where him buying a thornmail just doesn't do much for him compared to Nocturne.
Now he is good at isolated targets with his abilities, but so are all the other assassins in the game. Hell they could do it faster than Nocturne and get out quickly.
Now if you can keep winning with Nocturne just continue playing him. Don't stop playing any champion, if you win a lot with them.