Haven’t heard Gruff Rhys talk much, I do think James Graham amps up the accent a bit when he’s singing, especially when he rolls his Rs, I don’t mind it and it’s just slightly thickening his real accent as far as I can tell, but I could understand people being a bit put off by it.
I think in this case as I can pinpoint the accent he’s impersonating (north Dublin, usually stereotyped as gritty working class Dublin) which I don’t really hear in his speech (not sure where he’s from but I’m sure I read they all moved there to study) then it feels a bit more of a ‘dishonest’ affectation - however I don’t really mind it and think it’s a decent album.