Is there a shortcut for activating Low Power Mode in iOS 9?
Two Options:

Option 1:
Ask Siri to "enable low-power mode". This requires an Internet connection of some type because of Siri so if you are, in fact, using an iPhone without a data plan then you will need to be on wifi.

Option 2:
On iPhones featuring force-touch, you can force-touch the settings icon on the homescreen, navigate directly to battery and toggle low power mode there. Not much of a shortcut but it is something.