Yes, Nami has a tattoo on her shoulder. If you are doing the post-time skip, you can add it to the outfit. It isn’t hard at all to do. It is a very easy design, and you do not need to be an artist to make it great. It is a minor detail, and you can easily do your cosplay without it. You can also buy premade Nami tattoos if you do not want to paint or draw one yourself. I do not see a lot of people going as the tattooed Nami version, so it is totally up to you!