Something involving the doctor occurred to me awhile back, and others have probably thought of this previously, but here goes. The Doctor says everything and everyone is important, right? Well what's River Song's first rule? The Doctor lies. Opinions?
River’s not the only one who thinks this though! The Eleventh Doctor himself even changed his number one rule from time to time: instead of “Don’t wander off” it became “The Doctor lies”.
But I don’t necessarily think that means he’s lying specifically when he says that everyone is important! I think River is referring to when the Doctor lies for the sake of the greater good (or at least, what he perceives the greater good to be), like during the whole “my death is a fixed point in time” thing, and really the entirety of The Wedding of River Song.
So when it comes to River and her experiences with the Doctor: yeah, I don’t really blame her for associating the Doctor with lying.