[It's hot. It's unpleasantly hot. Chekov is from a climate where summer temperatures don't push much above the mid-seventies, and spaceships? Spaceships are only hot when something is very broken.
He's trying to be a good sport about the weather. He's even going to go to the beach in these great swim trunks and see if there are any distracting shenanigans to participate in. Good sport or not, he'll look a little wilted and, if the dome's artificial sunlight has the same effects on human skin as typical solar radiation, more than a little sunburned.]