It's in a well shaded area, and that green boat has been tarped outside there for three or more years with no ill effects. I've never noticed any sweating or deformations. The boats always feel cool to the touch when I take them out.
Just as an experiment this summer, I might take the temp inside and outside the tarp. If there's a significant difference, I might concoct some way to allow the ends to be more open and let a breeze through.
I doubt seriously the kayaks get anywhere near as hot as they might just sitting out in direct sunlight (on the ground, on a roof rack) on a hot, sunny day.