Most times squirrels will take shelter and stay in the shelter until the rain goes away but I have seen some squirrels out in the rain foraging for some food so yes some squirrels do come out in the rain while others may stay hidden in their shelters.
Squirrels may also leave their shelters and head for drier places if their shelter starts flooding out or something.
So yes it's normal to see some squirrels out during the rain although most of them will stay out of the rain to prevent getting wet.