In general, occasionally writing on skin with water-based inks is considered non-hazardous. Concern arises when people write on their skin frequently with permanent ink, or with inks that have certain coloring agents. If one is writing on skin on purpose, then using skin paint or skin ink is a far better choice, although even water-based inks may cause some skin irritation for some people.
The primary concern with writing on skin with permanent ink is that permanent ink may contain xylene. Xylene is a toxic substance, though toxicity is normally linked to inhalation. A mark or two of permanent ink on the skin because one is using a permanent pen is not likely to cause significant harm.
Your skin has pores and those pores let sweat out in order to keep the body cool they also absorb what ever is on the surface of the skin as well. So what ever toxins that are in ink will (((pour))) in to your (((pores))) now that's a (((poor))) decision to write on one's self. To answer you plainly yes it's bad for anyone to inscribe on one's self a tattoo would be much safer.