Cho's not that terrible. OK, so she didn't really know how to be sensitive to Harry (he had barely discussed Cedric's murder with anyone before then) when she and he went on their date in book five. Kind of makes me wonder if she saw him as a replacement for Cedric, rather than a person in his own right.

As for not talking to him in book six... that's not that unusual for a couple when they fall apart, particularly if they've never really had a relationship like that before. Any encounters after things fall apart are awkward for awhile - avoidance, or space to get over the other person, is a fairly natural tendency.