As far as I can tell there's no strict prerequisites for JET, either in terms of living in Japan, having experience with Japanese, teaching experience or having good grades. They look at the entire package.
That said, any of those things can help. Any thing related to Japan, teaching, volunteering, cultural exchange or that shows a particular skill (academic or otherwise) shows that you'd be a better candidate than you'd otherwise be. No single thing seems to be a deal-breaker, but the more experience you have that is relevant to the programme the better!
In my case I had terrible results for most of my degree, including multiple fails. But I did have a strong final year (High Distinction average, whatever that is in GPA). I also ticked a lot of the other boxes- I've had previous experience teaching and living in Japan, subsequent tutoring experience, a background in linguistics and a few other things that must have interested them.
Personally my grades weren't even mentioned in the interview, nor did I mention them in my SoP beyond saying "I've had some personal struggles that have helped me grow as a person ...." You might be asked, especially if you have a stricter panel than I did, but if you have a lot to offer to the programme it isn't a deal breaker.
* Melbourne Consulate Shortlist 2012 *
~ Shiga Prefectural ALT ~