Personally I believe that singers should always aim to sing in their native/natural accent/voice unless they happen to be very good mimics who can bring out the passion/emotion in a song whilst imitating someone else's accent.
I recently heard a recording of a singer who had an accent which was extraordinarily like that of a very well known singer on the British Folk Scene (I'm far too well brung up to mention any names!). As the well known singer has a very distinctive voice I found this very distracting. Perhaps I am misjudging the first singer - perhaps they were born and raised in the same street - but probably not!
I also remember, many years ago, hearing a singer, who sounded identical to Harry Cox, sing Harry Cox's songs in front of the great man himself! This could be construed as great flattery - but I just found it crass and embarrassing!!