I hiked the JMT over two weeks in August 2008. I had mosquito trouble in one spot- around Rush Creek at dusk. Once I thought it might rain at night. I used a bivy and it really only served to keep my bag clean and on the pad.
A month earlier on a family backpack from Red's Meadow to Shadow Lake, you couldn't breathe without inhaling mosquitos. Dozens would get in the tent whenever the kids entered or left.
The next year a friend hiking with his kids had several inches of freezing rain and hail near Donahue Pass.
Many years ago I did a 10 day August trip on/around the southern JMT and it rained every day for at least a couple hours and we had thunderstorms with strong winds, sleet, hail, and a little snow a few different nights.
So the answer is, it depends. If your dates are fixed and you plan to continue on no matter what the weather, I suggest preparing for the worst. If your dates are flexible and you're willing to bail if it gets miserable, then bugs are probably a bigger issue than sustained rain at night.
In any case, be prepared to survive a summer storm of freezing rain, sleet, or a foot of snow. This means technique as much as gear.