I find it really helps, a lot. Especially on a prolonged fast. That feeling of hunger and boredom at the 48, 72 or 96 hour I get, especially at dinner time, is completely erased after cannabis consumption.
I would say it entirely depends on the reason why you fast. If you are fasting to help your body recover, regenerate, and heal, I would say it wouldn’t be a great idea to put any smoke into your lungs.