My daughter's boyfriend has a tattoo on his forearm stating he is a Type1 diabetic.  His mom suggested it so that when he goes into diabetic shock and is in the ER unable to speak, medical professionals know why.

I don't have any tattoos.
Context matters here, which the OP grossly fails to understand.
Some tattoos can be a mortal sin, while others are not a sin.
My opinion is that tattoos reveal a worldly spirit to some degree, and there is nothing really original or creative about them. They're permanent human cattle tags identifying worldly spirits of persons at the time they received the tattoos, but that doesn't mean they're still that way. There are sincere converts to Catholicism who were previously neo-pagans with tats.

When I see Gen X, Millennials and younger with a bunch of tattoos, I see a walking joke.
The old school bikers with tattoos, yeah, they were the real deal but still caught up in the wrong spirit.

My daughter's boyfriend has a tattoo on his forearm stating he is a Type1 diabetic.  His mom suggested it so that when he goes into diabetic shock and is in the ER unable to speak, medical professionals know why.

People with health issues used to wear a bracelet that indicated what the issue was, fortunately it was easily removed. 

I don't like tattoos, I seriously doubt I would ever get one, but given that there are probable opinions either pro or con (mostly con), I cannot categorically condemn getting one.  Tattoos for purely medical purposes, such as a "Medic Alert"-type tattoo for diabetes, epilepsy, pacemaker, and so on, I can't see those as a problem (uniformity would be nice, so EMTs would know precisely where to look), and I also like the idea in the German military (IIRC during WWII) of tattooing a soldier's blood type under his arm (again, one uniform place).  I've also heard of putting a very small tattoo on the feet of identical twins, a small dot or something, to be able to tell them apart, especially useful for caregivers.  It could be placed inconspicuously so that it would be barely noticeable later in life.  I could get behind that too.  But a tattoo just for the sake of having one?  I'll pass.

Piercings seem unproblematical, in that they will gradually "heal up" if the jewelry, brad, etc., is removed and left out for a time.  So they're not permanent mutilations.  But I don't have any of those either, and would not want one.

People with health issues used to wear a bracelet that indicated what the issue was, fortunately it was easily removed.

He has a metal allergy hence the tattoo.

Yeah, I'm not just seeing it.  Based on what principle of moral theology is any tattoo grave matter?  I could see the gross excesses that some people go to, due to disfigurement of the body ... along with those gross piercings.  But to make an analogy with piercing, I can't see how a woman piercing her ears for some earrings would be grave matter, whereas those bizarre tongue piercings and nose piercings definitely cross a line.  I would think it's the same for a tattoo.  If it doesn't grossly disfigure the body, I can't imagine a principle of moral theology that would make it grave matter or even venial matter.  And probably some tattoos would be serious depending on what they were.  But a small cross tattooed on someone's upper arm, for instance, I just can't see it.

Yeah, I'm not just seeing it.  Based on what principle of moral theology is any tattoo grave matter?  I could see the gross excesses that some people go to, due to disfigurement of the body ... along with those gross piercings.  But to make an analogy with piercing, I can't see how a woman piercing her ears for some earrings would be grave matter, whereas those bizarre tongue piercings and nose piercings definitely cross a line.  I would think it's the same for a tattoo.  If it doesn't grossly disfigure the body, I can't imagine a principle of moral theology that would make it grave matter or even venial matter.  And probably some tattoos would be serious depending on what they were.  But a small cross tattooed on someone's upper arm, for instance, I just can't see it.

With piercings, it would be more a potential sin against modesty (in this case, adorning oneself flamboyantly and gratuitously, not a question of sɛҳuąƖ impurity), and a sin of pride and vanity (again, the flamboyance and "look at me!" factor, in the case of exaggerated and too-numerous piercings, or piercings in outrageous places), than a mutilation of the Temple of the Holy Ghost.  As I said above, piercings will heal back up in short order, if you leave them alone and don't put anything back through them.  The piercing, all by itself, is probably so trivial, as to be no sin at all.  Ultimately, I would defer to a traditional Catholic priest on that last point.