the body is made out of dust and returns to dust.
why toss out a marriage over an argument about dust?
i understand your position, and i wouldn't be for a tattoo in the first place, and as i understand (though i'm not from that culture) ta moko is a symbol for heritage and ancestry - so i agree with you; we are adopted through Christ into a different heritage than the heritage of the flesh. ta moko is a symbol of integrity too; if anything can be received with thanksgiving and used for the glory of God, i'm for it! we are free in Christ. maybe together you can ascribe meaning to this that honors her culture and also the new heritage that's given to you; the family of the kingdom.
what's more important than the skin, is the heart.