While lip syncing, making it look good is more important than making it sound good. I am not saying I agree with lip syncing at all, but if it is going to be done, then logic suggests that she concentrates more on selling the look than making sure the vocal that no one hears will sound decent. There is no if she can sing in the studio...we have all heard here songs on the radio etc... she can sing well enough when it counts (when it counts being in the record execs minds of course), and as I said, she is known to sing one or a few songs live at her concerts. The reason she doesn't produce a similar sound while dancing etc is because her concentration is elsewhere. Lip syncing is usually attempted to be justified by the fact that pop artists "can't dance and put on a show the way they do and sing at the same time, so they lip sync to give the whole performace package"...or some shit. I know this is a load of crap, because any real performer like Pink for example, CAN do it, and really most people going to the shows would rather hear her actually sing than dance, but meh, that is how they will try and justify it to you. I guess on the other hand, Michael Jackson was known to lip sync for performance purposes, and if anyone claims he couldn't sing then they are out of their fucking minds.
The fact it was never meant to be heard does make a difference. It is like saying an artists sketches and concept work needs to be of the same standard as their finished product, just in case someone sees it. Bollocks.