Answer:
By telling her how getting a tattoo would not affect your moral principles.
Step-by-step explanation:
The one way to persuade your mother to let you get a tattoo by using the rhetorical device of Ethos would be telling her how getting a tattoo would not affect your moral and individual principles. Ethos as a rhetorical device means to convince the people through the speaker's character and credibility.
Likewise, this can be used with your mother by telling and proving her how strong and resolute you are in your decisions. Just as most people view people with tattoos as delinquents and bad people, your mother must have the same thoughts as well. However, you can change her social-construed views about people with tattoos through your refined character and credibility.