Well, there’s the problem right there. You’re expecting the series to a flat out comedy instead of a drama with comic undertones because that’s what the movie is.
What you apparently don’t know is that Joss Whedon never intended for Buffy the Vampire Slayer to be a flat-out comedy. Only when he brought the concept to television was he able to realize his true vision, a teen drama/horror/action/comedy.
The problem with basing a long-running series on a one-note joke like the idea of a High-School Cheerleader who slays vampires is eventually the joke stops being funny and when that happens, you’re left with nothing.
Besides, other than one episode in the first season the Buffy of the series has not been a cheerleader or anything close to a cheerleader.
In fact, the only real similarities between film and series are the title, the name of the main character and the basic concept. I thought the movie was okay but nothing special. I think the series is excellent, sometimes bad but frequently brilliant.