They made Heather look worse than she really was...
Yes, in the book too Heather was a clueless little brat who cared more about being popular than anything else, including the one person who had really been her friend from the beginning. And yes, it was pathetic that she would blow Melinda off the way she did and then expect Melinda to help her with her little project the moment the Marthas started dissing her. But let's be honest - they were only 14-15 years old, and a lot of people are like that at that age. High school is such a sink-and-swim place in terms of popularity, and if you don't have friends, it can be absolutely miserable (I am speaking from experience - I was the girl who ate lunch alone every day and never talked to anyone ever). Just because someone is like that at that age doesn't mean s/he is a bad person. It's more immaturity than anything else. Now, if they were 10 years older and Heather was acting like that, THAT would be a problem, but at 14? I thought the movie purposely made us see Heather as the biggest loser in the world and a total b*tch, which wasn't necessarily fair.
I will say though, when she told Melinda that she didn't want to eat lunch with her anymore and basically blew her off because she was depressed - THAT was harsh. A real friend would actually care that you're depressed and try to help you, not just ditch you for superficial reasons.
RESPECT MY AUTHORITAH! - Eric Cartman