Really don't get why are equating authenticity with matching prevailing norms.
I'd say in most cases someone who uncritically (or deliberately) attempts to match up to some 'prevailing norm' (as many do of course) is going to be quite inauthentic (alienated) at least some of the time.
Well you can be authentic a) to yourself (your current behaviour fits well with previous behaviour) or b) to others (your behaviour fits with surrounding behaviour), as, by my earlier definition, authenticity is degree of fit to context (either temporal, which would load on personal behaviour, or spatial, which would load on fit with others' behaviour). Alienation would involve a tension between a) and b) (note that this tension exists because of the (assumed) consistency of personality within the person afflicted).
That said, I would expect most people to have little difficulty inadvertantly matching prevailing norms, seeing as these norms are embodied in people in the first place.
The middle class, white rasta is behaving inauthentically with regard to others (his behaviour is unusual in the immediate social context and cannot be explained by what is known by others of his past behaviour), but, I suppose, might be authentic to himself (if it has some fit with his previous internal states). Similarly, Madonna's inauthentic (unpredictable) behaviour could be said to be personally authentic, as
she is wont to behave ostensibly inauthentically in the way that she does.
I think theme bars for the most part quite clearly set out to celebrate an exaggerated and simplified lcd caricature that's amusing / reassuring. It's not really claiming to be true to life is it?
I suppose there is that, but 'authentic' behaviour by the other is usually expected to satisfy either
a (fit with previous behaviour) or
b (fit with immediate context). Simplification and caricature emphasises both fits by reducing incongruous behaviours and emphasising congruent ones, as well as, with some poetic license, recreating the wider context with reference to which its authenticity is to be judged.