Everybody Loves to Hate Cho
Let's face it... if there is ANY character that a Potter fan loves to hate, after Dolores Umbridge, Vernon Dursley and Ginny, it's probably Cho Chang. So why her? Why this poor girl? Let's go over some of her character and see what we get.
First, we have how Cho is introduced in Prisoner, and is treated as a crush till Goblet of Fire, where she is presented as Cedric's girlfriend. This is effectively a sort of first strike against Cho, as readers may go "Harry's sweet, why are you going with Cedric?" Though in truth, this sets up the real reason people probably have to hate Cho: the tea shop. Now, after Goblet of Fire, Cedric dies. It's pretty obvious in Order of the Phoenix that Cho is slowly moving towards Harry, and the reader naturally may clap a bit for Harry, because things so rarely go his way. This is where Rowling plays a bit of "kick the dog" as Cho is only moving towards Harry as a way of coping over Cedric. At this point, Harry is too immature to realize this, though a few readers may have seen it coming. Combined with Cho's whiny-whistful behavior at the tea shop, and Harry's (and the reader's) expectations, it becomes hard not to be irritated at her in the slightest. Even worse, she is the friend of the person that ratted out Dumbledore's Army (and was the person in the film). Readers would be justifiably angry at Cho, and grumpy because her behavior is well, that of a girl that was thrust into unreasonable circumstances and is coping and recovering, when the readers wanted someone tougher, meaner and a bit more mature.
That being said, it's hard to really justifiably be angry at Cho, but the problem of why readers would still be angry is because after this circumstance, she vanishes from the narrative. She never gets a chance at redemption, which is something that Wormtail actually got in the final book, and something where Neville really shone as his growth as a wizard was on full display, even killing Nagini with the Sword of Gryffindor. Cho as a Harry fangirl would have been insulting, and really, we never get to see her beyond that romantic object in the writings, as she was either "Cedric's girlfriend" or "Harry's crush" till the tea incident where she became "whiny pratt". Contrast this to Ginny and Hermione, who are both set up as actual human beings with goals and talents distinguishable from their roles as love interests. Or to the Patil twins, who were given some characteristics as human beings aside from "female love interest" and were never even truly treated as a love interest but more like normal young ladies.
This is probably the real reason why Cho is so easy to hate. She's only half a person, and that half we see is whiny, unlikable and was only in the wrong.