Is it bad if a kid draws or writes on their skin and doesn't wash it off for days?

That is a sign that they may be angry or lacking proper respect for themselves. They should be told until they believe it (through Words and actions) that they are a good person, deserve respect, and don't deserve any kind of mistreatment from themselves or others. Also, help the child clean this off, make it clear that ethical behavior is required in every way, and this is not an acceptable habit. Show the child through role modeling how to behave, speak, and take care of his body.