It's another lucky pervert joke, but using the clear plastic sheet as a metonym for a screen was a nice touch. His viewpoint of the pantsu is framed, like on a phone or camera viewfinder, or a work of 2d art. There's a tradition of voyeuristic images in paintings and photos. Combine that with how we didn't see her crotch, it makes this a more subtle and artful lucky pervert moment.
Exactly; Characters usually refer to panties as them, but he didn't say he saw them, he said he saw it.
It's hot, if her shirt is soaked through so would be a tiny cloth hidden under a stuffy skirt.