She always plays her characters as slow, dim-witted retards. It must be an annoying stereotype for her to play, if that's what the directors all demand from her, as I think she's probably quite intelligent and professional in real life.
She's like a younger, female, Jim Broadbent - always plays a mong.
Actually, there was nothing especially 'retarded' about her role in that, or the way she played it. Remember that scene with DeNiro pretending to be her drama teacher? That was actually improvised by the two of them. Doesn't an actor have to be kind of on top of his/her game to hold the screen with an actor of that caliber?
Secondly, being 'on drugs' is pretty tough to pull off when you are working with one of the most well-regarded directors of modern times. Don't you think SOMEONE involved with the production would have pulled her aside and said 'cut out whatever you are doing!"?
Secondly, if you watch "Natural Born Killers", you will see an actress with whip-smart reactions to the other folks around her - notice how easily she karate-kicks the bigger men who threaten her in that opening scene in the bar. She doesn't come across as even REMOTELY 'retarded' there!