OF course she is. She just murdered hundreds, possibly thousands of the kingdom's loyal subjects in order to "take out the trash". (trash that she put in power herself) But don't believe me, and don't believe the look in Jamie's eyes when he sees her afterwards, just go back a few episodes and watch Bran's decades-spanning vision. He sees the mad king shout "
burn them all!" and the next scene is the catacombs under the Sept going all-assplody with Cersei's wildfire. Can't get much more direct a comparison than that now, can we.
The real question is not whether she is the new mad king. (she is). The real question is whether she is a Romulan. (she is)