Someone else’s scars are none of your business and your opinion about the mental health of people you don’t know is completely irrelevant. The sad fact is that there’s enough vocal, content to uphold the stigma against mental illness that people who have suffered from self-injury often feel like the HAVE to get tattoos, plastic surgery, wear long or sleeves and pants just to be able to go about their lives without being harassed and judged.
Choosing to self harm is due to a mental illness or condition that causes people to crave attention. Covering up the scars with tattoos isn't going to help anyone deal with their problems. If they need to do so in order to not cause further questions later in life, that is up to them.
If the person who has self-harm scars wants to get tattoos to cover them up then they should. If they don't want to, or dislike tattoos, then that should be okay as well. More importantly, people should learn to be more accepting of the scars and eliminate the stigma around them.
They have to bare them forever so that normal people can tell the type of person they are dealing with. I certainly do not want to entrust the care of my child to someone who can't stop cutting themselves. They did it, they can display it. Makes for an interesting conversation piece.