i think it is Very Important to consider the fact that matthew would most likely have tattoos of his own.  Alfred sees them all up close for the first time and is sort of mesmerized by them bc theyre all very elaborate and matthews like yeah i designed them and had a buddy do the actual tattooing and alfred is really struck by how artistic matthew is?  he clearly is creative and while alfred doesnt know if the images themselves represent important things to matthew, its clear that the artistic process itself is a critical part of who he is.