I've been to Whistler 5 times and it has rained every time except one. Doesn't matter if I go early, middle, or late in the season, I bring the rain.
It's still fun but a different kind of fun. Bring a rain jacket, some extra brake pads, and the stickiest tires you can find. Also, call and find out what trails are open, they close down a lot of the groomed good stuff when it gets really wet.
Depending on how wet it is, they sometimes close down various trails. Like Joyride, Whistler's DH, Schleyer, etc - the more "dangerous" ones. I've been there when it's wet and A-line would be shut down and the only thing open is B-line.
Garbanzo can be a mess when it's wet and muddy. The ladder sidewalk section on Duffman can be very slippery.
i generally think the shore is a much better ride in the wet. lots of good whistler stuff gets closed in the rain, and the faster trails are a silty spray fest. a couple a-lines & you're coated in grit. there are lots of well drained & armored shore trails that are good times in the rain.
Rode whistler in MId july this year and it was hella wet and fun. Garbonza was close a couple of times due to the conditions but wjhen it open backed up, it was the ****tttznick when wet. I would defintely bring googles, rode w/ out googles once and i almost killed myself on crabapple hits going mach10 when raining.
if it's raining drive the extra 25 minutes to pemberton and ride....the drainage is killer thanks to the very course nature of the soil, almost sandlike....you can ride there when it's raining and not even get muddy.....slippery sure, but not overly muddy....