dumb? u mean stupid? she's smart compared with others except detectives and scientists. she's just a high school girl, not genius =0=
btw if u mean dumb because she cannot figure Conan is Shinichi then i think that is Gosho's plan, i believe he is saving this for the right time, Ran will find the true when Gosho wants her too, now's not the time, it's not her fault. I think Gosho already has a plan about this and he wants this to be special.