It seems to me phoney and unnecessary to assume an accent other than one's own for a song that comes from somewhere where the accent is a lot different (for example someone from England singing a song from the southern US or Ireland) if the words are generally the same in one's own sort of English. (I used to do it myself in my younger days but I like to think I am wiser now.)

However it seem to me equally phoney to sing a song that is full of Scots words in RP.

One particular bugbear of mine is to hear someone singing in an English accent but with the odd "doon", "frae", "hae" etc here and there. Either those words should be replaced by "down", "from", "have" etc OR the whole song should be sung in Scots. Even if that is a sort of generic Scots, not exactly authentic for any specific part of Scotland, it seems to me a lot better than Scots words in an English accent.