The question of tatoos whether permissable or not seems to be contradictory as far as Rabbis are concerned. The Rabbis say no but G-D says yes. G-D took it upon himself to tatoo the first son of Adam, Cain bore the mark that G-D marked him with so how can tatoos be contradictory to our religion if it had G-D initating the first mark on man.
The Torah says "HaShem placed a sign on Kayin". You interpret this as a tatoo. I have not found the specific term tatoo in commentaries or midrash. There is an explanation that the sign was letters (or a letter) carved into Kayin's forhead. That could indicate a tatoo, or a scar, or an indentation in the bone. So the question you have asked depends on a very specific interprertation of a specific position in the midrash. There are several other interpretation of "sign" e.g. a dog, a horn growing on his forehead, etc. Even if there were a specific midrash,. your question assumes that somehow HaShem is bound by the laws of the Torah, or at least that anything HaShem does is definitively good, and the Torah will not (can not?) contradict it. This contention reminds me of the story of Reb Levi of Berditchev, the great chassidic leader, who, on Yom Kippur said, "You HaShem, keep the holy Torah, and it is forbbiden by the Torah to do melachah on Yom Kippur" Therefore, You may not write on Yom Kippur, unless it is for the purpose of pikuach nefesh, saving a life, which is permitted. Therefor, You may only inscribe the Jewish people in the Book of Life, for any other writing would be against the Torah". However, there is no compelling reason to say that G-d is bound by the laws of the Torah, nor that something that is forbidden is "evil" even when not performed by a Jew. HaShem is outside the bounds of law and ethics, even if HaShem sometimes subjects Himself to those rules, that is to teach lessons to his people, but not because He is bound by them. Generally we cannot learn halacha from anything that happened before the giving of the Torah, so what happened to Kayin is not a binding halachic factor. We find many instances of the Avot not keeping the halacha, and although many reasons are given, the fact remains that before matan Torah the mitzvot existed in a different way.