It's really not a huge deal to be Prom King or Prom Queen. Naturally, it's a huge honor because you're pretty much being bestowed as most popular / most well liked at the school, but its not HUGE.
Prom IS a huge deal though. When I told people I wasn't going because they wouldn't let my boyfriend go, they told me to just dump him and find another prom date because it's a once in a lifetime opportunity. So, having a prom date for 1 night is apparently more valuable than a relationship that's over a year.
It's extremely elaborate though. Salons are BOOKED for the month before prom, and pretty busy for the weeks before that. Every girl ever getting her hair and fake nails done, people spend over 200 dollars on dresses, it's such an expensive process. Boys rent tuxes and get haircuts and that's their big deal there. Tickets for my school this year are 60 dollars apiece.
Also, spray tans, spray tans everywhere.
For an idea of how much money I'm saving by not going, I managed to book a round trip flight to NC to see my boyfriend for a weekend, and visit a huge zoo, and a big amusement park. Well, I might overspend by like 20 bucks, but it's that insane for one night of your life.
So, as a tl;dr, I would say Prom King/Queen is a big deal like portrayed in the movies, but Prom in general really is.