In principle, the Church does not object to tattooing. I don't know the context to which you refer when you say you were told "excessive tattooing" could be a mortal sin (assuming that there was full knowledge and full and free consent, in addition to the grave matter of the action), but it was possible that the person referred to the principle that one cannot purposely endanger one's health through cosmetic enhancements. So long as there is not a danger to your health, and so long as you are not spending money on tattooing that should be spent on personal obligations, you are free to receive tattoos.
Recommended reading:
Tattoos by Jimmy Akin