Misa wasn't stupid. She knew that. She also knew that nobody else did.
Misa wasn't a genius like L, (who quantified everything in life with percentages.) she wasn't like Light, (who scored a hundred percent on the ToHo university exam.). Misa wasn't a genius; but she wasn't stupid. Misa knew her greatest strength lied in her acting. Misa knew people. Misa knew what people wanted, what they thought, why they did the things they did. She knew what people wanted to see in her, what they were going to do next and most importantly; Misa knew herself. Misa knew what she wanted and how to get it, and she wasn't afraid to do whatever was necessary to achieve her goals. And when Misa looked at Kira who was too busy terrorizing the world to notice the smirks she shot at his back, when Misa realized that the stalkerish, perverted L couldn't be bothered to look up her high school test scores, when Misa saw the condescending leers the men of the task force shot her Misa laughed and laughed and laughed.
After Misa first picked up the Death Note; after Rem first explained to her what it was - she was ecstatic. She thought (yes, here, my new challenge, come, welcome). She thought she had finally found a way to stave off boredom for more than a few weeks. That's why she placed that hair on that manila envelope, that's why she made the eye deal. (After all what fun is a game that you're not present for?) Because she had broken many hearts and many bones in her lifetime but the idea, the very thought, of doing this all to Kira, to L, to the best the world had to offer - well no one could pass that up.
Misa had learned a long time ago that for every talent a person possessed they also had an equally damaging flaw - and Misa learned through her newest playthings that genius' great flaw was foolishness. It was because of this that she glad she wasn't a genius - after all fools always lose and Misa-Misa was born to win.