If you are talking about the dumbest anime girl there’s no way you can miss Misa Amane. Nothing is more annoying than Misa Amane in Death Note. She’s childish, impulsive, and exasperatingly hyperactive. Misa also has shinigami eyes, which allow her to see a person’s name and lifetime above their head, making using the Death Note to murder them much easier.
Her actions as the Second Kira are extremely unreliable, resulting in her arrest and detention on suspicion of being the Second Kira. Misa’s yandere-like obsession with Light led to her demise, despite the fact that it was most likely the result of an unresolved concussion.
Misa commits suicide after Light’s death, as hinted at in the anime and depicted in the manga. Misa is just plain stupid, aside from willingly making everything in her life about a man.