I think you’re the only person who finds it sexist to label Cersei “mad” — as others have said, male characters on the show have been called the same. There is an obvious, intentional parallel that the showrunners are drawing between Cersei and the Mad King. She did everything short of chant “burn them all” as the Sept of Baelor crumbled.
As for Tyrion, he employed wildfire as a defense while at war, not to murder hundreds of innocent and unsuspecting citizens WHO WERE GATHERED TO WITNESS A TRIAL FOR CERSEI’S CRIMES. You can’t get to a much higher level of dictator than mass-murdering the people who are trying to bring you to justice.
To put this another way, Cersei fancies herself as Tywin’s equal. From everything we saw of Tywin’s character, he would never pull a stunt like she did with the Sept — BUT if he did, you can bet that everyone would be calling him completely insane as well.
Cersei is a madwoman, and everyone knows it.