I did the JMT NOBO in late August last year - 2020 (Aug 24 - Sep 2) and I had literally no issues with mosquitos at all. Actually, it was kind of odd. I brought a spray can of OFF - Deep Woods Dry spray and I never used it once. I actually gave it to a south-bound hiker. I think it all depends if we get a heavy snow and wet rain season - mosquitos will be a problem. Last year was extremely dry - no mosquito problem.
As for the forest fires, I started at Horseshoe meadows and the Sequoia complex fire blew smoke northwards and it was really bad. Whitney was awful, and crabtree meadows was a snow storm of ash. As I passed Forester Pass, the smoke was never a problem for the remainder of the hike. I finished just as the Creek Fire started - got lucky.
Forest fires will be an issue this year if we get another dry weather pattern - hopefully more snow will arrive in the sierras!
Good Luck! I too am doing the JMT again this summer and NOBO is for sure a great way to go.