He can shift to any form, with or without the tats. He could shape shift into one big tattoo if he desired. Also the "real" Maui had no tats. The tattoos were added by Disney to play into the Dwayne Johnson fantasy. Disney likes to use "star power" to sell their movies as they did with Robin Williams and have done with many other stars.So long as he stays in the shape he shifted to. His tats stay on his human form .