Is it worth modding an external audio jack to a camcorder?
I would say the most likely cause of the LoFi audio is the data compression of the audio to get it small, or alternately the AD conversion, possibly the acoustic properties of where the mic is housed. 

If you like the look of the footage and want to capture better audio I would recommend using an external recorder like a Zoom H1, or even just a phone that you have, and sync the audio and video in post