I haven't heard of anyone getting skin cancer from drawing on themselves.
As for it being bad...well when you use permanent marker/ink there is a substance that the marker contains and it is called xylene. Xylene is considered toxic. I believe it's only toxic when inhaled but it can be dangerous if ingested if one intentionally swallows something that contains xylene in it. I believe xylene is used to stain furniture. I believe it's a clear substance. It can cause irritation to the skin especially if one has broken the skin open when writing on themselves and if that happens it's hard to remove the ink. The types of irritation just depends on how sensitive someone is. I know that when I was younger I used to do it almost all the time.
Hopefully this helped and good luck.
