Can the iOS 15 weather app change to have the old compact look(GUI)?
No, Apple doesn't offer a way to customize the Weather app (except for choosing a city to display).
I'm extremely disappointed by this because there's some info that people in California, who coded the app, might want to see at the top, like Air Quality, that I couldn't care less about.
There's many different 3rd-party apps that provide a myriad of different layouts and you might find those more to your liking.