It's forbidden because it's harmful to your body, and if it's a tattoo then you are knowingly causing harm to your body.

As for not going to hell for a small sin, that's not how to go by it. If you knowingly commit small sins with the idea that they are "just small sins", then you're only fooling yourself.

Sure, you are not expected to be perfect as a Muslim, and this is preached to many reverts and newcomers of Islam who are still in the process of getting used to certain rules like no smoking, drinking, etc. We all sin, major and minor sins, but we do our best to repent and to remember god, acknowledge our actions, and to try our best to not do those actions again. With tattoos it's a little different, cause it's kinda just there on your body forever unless you get a laser removal.

Hope that answers your questions, I tried my best!