Cartoon angel and devil tattoos represent an individual's conscience or a naughty-and-nice personality type. They likely originate in shoulder angel and shoulder devil depictions in comics that have the same meanings. Good versus evil is the overarching theme of these tattoos.
Comic strips often show a character with an angel sitting on one shoulder and a devil sitting on the other. The implication is that the character's angelic traits are in conflict with the character's devilish traits. The angel tries to convince the character to do something good while the devil instigates bad behavior.
A classic angel and devil example is a person on a diet who is tempted by "sinful" foods. The shoulder devil might be shown holding a rich dessert while the angel is shown holding a piece of fruit.