Getting usable sound from a camera mounted microphone isn’t really that difficult. You just need to choose the right one for the job. I’ve seen camera operators attach all kinds of goofy mics to their cameras – and often the recording is pretty much trash.
So, which mic do I use for a camera mic? Well, for me, it’s the Sennheiser ME64. Check out Gear Lab #3: Camera Microphones, to find out why.