Jump Starting a car in the rain is perfectly fine as I've done it many times.
The Jumper cable ends are insulated anyway to prevent your from getting shocked so as long as you don't make contact with the metal ends of the Jumper cables then you should be fine.
Just avoid also touching any metal on the vehicle while Jump Starting it in the rain and you'll be fine.
Even if you did get a shock from the battery you're unlikely to be harmed because of the low voltage of the battery.