Why is everyone sweating so much?

Sweating was IN in the 1980's.

See- Flashdance (1983), Body Heat (1981), Jane Fonda workout videos,watch a standard A-Team building montage, "Physical" video by Olivia Newton John, Rocky II, III, and IV

Sweating was never so in than the decade of the 1980's.

Most places didn't have air conditioning. My HS didn't have it. Almost no US Army building had A/C so I'd guess other branches were the same. It was still a luxury thing in cars.

Typical, overhydrated, pinko-Hollywood types... that was probably before the drought, you see.

I guess they were hot.

A former Tomcat flight officer named Commander Ward Carroll fact checked Top Gun for Digital Spy back in 2018, and he said that in real life, those spaces on the ship are usually freezing cold to protect the electronics. So it would be highly unlikely that the CATCC controller at the beginning of the movie would be sweating so profusely. Also, the only logical explanation for why Maverick and Goose would be sweating like they did in the cockpit is that if the pilot had the environment control system (ECS) jacked up uncomfortably high.

https://www.digitalspy.com/movies/a863515/top-gun-mistakes-technical-blunders/