Writing on your skin with Sharpie is not considered good but I do it all the time and so far nothing bad has happened to me.
Supposedly you could get ink poisoning from the Sharpie if the ink from the Sharpie leeches into your skin and bloodstream.
But most times you'll just get sick if that happens and if you do then see a doctor and treatment can help.
I try to avoid writing on my skin with Sharpie or anything else but I just have a habit of doing it and it has been going on for years since I was a kid.
Even as a toddler I remember writing on my skin with non toxic markers and I have pictures of myself at 2 years old with marker marks on my skin.
It would sometimes take a week for that marker to fully come off but it eventually did.
So if possible I would avoid writing on your skin with Sharpie but it's unlikely to harm you but some people may be allergic to that Sharpie ink and so it might affect one person and not the other.