|Quoting viv (Reply 1):|
I try to get exposure, lighting and colour temperature right when I shoot, not in post-processing.
This is the best way to start a good photo.
However, to reply to your question if your shot was not that good, you will never get the same (usually good) result with Camera Raw on a JPEG image as with a RAW image. Sorry to be a bit technical... A JPEG image is digitally encoded using 8 bits and is already compressed by your camera body, a RAW image is encoded most of the time using 12 bits (14 bits on high-end cameras) with no compression. So, even if ACR permits to act on JPEG images, it is only "trying" to apply corrections on an image which contains a lot less informations than a RAW image, so the quality and the result can only suffer.
Hope it helps