Any thing done against God may be a sin, but the sin comes from the heart not from the results of a needle and ink in our skin alone.
For you, getting a tattoo might be a sin because of where you with God, but not every person is the same as you nor in the same place with God as you. Be very slow to point fingers or to speak words at another person calling things sin for that person. You do not know his or her heart. God does!
"Judge not, that ye be not judged.
For with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again.
And why beholdest thou the mote that is in thy brother's eye, but considerest not the beam that is in thine own eye?
Or how wilt thou say to thy brother, Let me pull out the mote out of thine eye; and, behold, a beam is in thine own eye?
Thou hypocrite, first cast out the beam out of thine own eye; and then shalt thou see clearly to cast out the mote out of thy brother's eye." Matt 7:1-5