We visited Sedans a few years ago in December, a couple of weeks before Christmas. There were no crowds nor traffic, but someone we met there said it was definitely a lull before tourist season started again!
I'm not sure what was being said about the hippie shops, that's not why we went there. We took day trips all over the place, and there is a lot to See! Lots of great hiking as well. When we went, it was cooler in the morning, but warmed up pretty well by mid-day. The GC was about 30 when we got there.
You may want to adjust your timing a little if you can. Not saying it would "suck" if you didn't, but why go when it's crowded if you don't have to?