Don't know that I'd use American Psycho as my go-to example of an ambiguous ending, but that's beside the point.
The spinning top in Inception can be seen as a tacked on, ambiguous, maybe-twist, and I can understand why it might be unsatisfying as such, but I don't think that's the only way to read that moment. Rather than hinting at the "is it all a dream?" question, I think the spinning top has more to do with Cobb's catharsis. We don't see it fall because he doesn't see it fall, which calls attention to the fact he willfully walked away. He's at peace with his current situation and his current location, which is what he's longed for since the beginning of the film