Mulligans
General Mulligans
Generally, this deck will want to accelerate Mana with ramp and Innervate, the faster it can start playing its big threats, the better. Keeping Jade Idol is an open debate, but generally less important than ramping.
Aggro Mulligans
Against aggro, Jade Druid will usually keep one or even both Wrath's to deal with opponents early threats, and maybe one Swipe, though that can also be a bit slow.
Control Mulligans
If the hand is already good, has some ramp and Innervate in it, keep Aya. On or before curve, very powerful option.
This is a pretty standard Jade Druid list that is used in Switters’ Anti-Guide: How to Beat Jade Druid article.