Short answer is that the person or people who are supposed to be singing ought to be singing whether on stage or on a recording. Miming is a huge no-no. Anyone who remembers
Milli Vanilli knows the score on that one.
The longer answer is that it's not a black and white division between singing and not singing. Pretty much every singer uses some kinds of effects, whether that be a touch of reverb and a little EQ tweaking all the way through to running the vocals through a distortion pedal so they are no longer in a natural human vocal range or using a great deal of pitch correction to hide an inability to sing in tune. It would be unrealistic to expect every singer to ditch all the effects and rely solely on their natural voice, and it would deny us all a great deal of genuine musical expression.
I'm also somewhat surprised that people tend to consider lip syncing or miming to generally dreadful but are much more tolerant of electronic music artists doing 'faked up' performances. Perhaps it's not such a big deal whether an artist is faking it, it's whether they get caught or not?