Started Off Promising

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They had always been like that. CHRIS didn't say it until the last part (but it was very obvious to me), and we didn't see that much of ASHLEY at the beginning.

Agreed - to counter IMDB_Vitt's response, Ashley's first appearance, when that stoner guy falls off his skateboard, she smiles and asks him if he's OK and hands his book back to him. Moments later, she's also nice, not to mention quick-witted, to Brian/Ryan when he bumps into her, causes her to mess up her lipstick and can only say "sorry about your mouth" - I considered this niceness and humour to be part of her appeal and the reason he fell for her. I also got the impression she was getting on well with Marla Sokoloff's character during their double date.
It's completely at odds with the person we see later in the movie, being horrible to the waitress and actually to everyone outside her clique that she considers beneath her, and, as stated in the OP, generally acting like a dumb bimbo.

While Chris was a jerk from the beginning, he also came across much more intelligent and cunning initially than he appeared to be later in the movie. His motive for Marla seemed contradictory too, as at first it seems like he's intrigued by her BECAUSE she's not interested in him, and that if he were to win her over, he'd want to do so for good and not just for one night - his "nail and bail" approach just doesn't go with the time and effort he takes chasing her.

Other threads have commented on the lack of continuity in other areas of this movie, and it's a shame. I guess they had to change Ashley's personality in order for Brian/Ryan to fall out of love with her and thus set up the triangle with Marla's character.