Well that's where you're wrong. Actually you CAN discern quite a bit from the cover.
You missed the whole point....walking down street there is no way you would have recognized Jeffrey Dahmer as a Cannibal. Ted Bundy did not look like a serial killer...so yes you can say that, but even police gave back to Jeffrey Dahmer a victim, and he killed and ate him later.
So yes you may discern a few things about someone by a tattoo, or their past way of thinking, since you may not know when they got it. A sailer may have got a tattoo 35 years ago, but may now be a hedge fund manager....etc! Some go to trouble of removing a tattoo, some don’t bother. To determine if they are a criminal is much harder to discern.