I did hear a sermon about this.
Fr. Crane gave one, just at the time I was considering getting a small one...(back in my younger, crazier days). It IS a mortal sin. And the reasoning is: the body is the temple of the Holy Ghost. Any tattoos would be a desecration of it. Thus, it's a mortal sin. I can't remember if he applied that same precept to multiple body piercings or not, and how women getting their ears pierced would pertain to it, but I DO remember that part quite clearly!