Stupidity: Medusa is the queen of the secret kingdom on the moon. She knows that they are very secretive and keep hidden from the humans on Earth who would have no idea who she is or that there are secret people on the moon. And yet in Hawaii, she walks up to an ATM and tries to demand by voice commands it give her money, saying, "I am Medusa of Attilan, and I need money." It is extremely stupid of her to think a random ATM in Hawaii would know who the secret queen of a secret moon base is.
Through the Valley of Shadows - S2-E12
Stupidity: Burnham and Spock find one person alive from the otherwise-dead crew, miraculously. They know Control can infect humans, they know how advanced it is, the entire setup screams "trap", and yet they don't even question whether Gant even might be under Control's...well, control. A simple scan would have turned up anomalies and they could have avoided what followed.