Last night my mom was saying how a person who ran for mayor played “dirty”. I told her my friend agreed with her. Did I commit a venial or mortal sin? Should I go to confession?
What commandment do you believe you broke?
From here: if what you have revealed to someone about a third party is true, you are not guilty of calumny. If the person you revealed it to has no right to that information, however, you may be guilty of detraction, which involves “revealing something about another that is true but harmful to that person’s reputation”.
In this case, I don’t see any problem You probably felt happy that someone else whom you hold in esteem shared your mom’s opinion.
Since we are not required to confess venial sins and imperfections, but nothing stops us from doing so and the saints recommend it, do feel free to mention it in your next confession if you want