Watching the anime made most of us think Misa was some stupid ditz, but as it turns out, it was really Light made a really bad judgment call by having Misa send that last tape with the message he told her to send to Sakura TV for the police to see. Were it not for that action, L would not have found incriminating evidence to use against her like that, and all she did was follow Light's orders (other than seeing him spontaneously which was all just very emotionally driven decision-making on Misa's part). Light calls her an idiot, but the dumbest thing she has done so far was take orders from Light. Whether she saw him earlier or not, she was already cooked.