I'll maybe need to duck here but to me I don't think Adele sounds like she is particularly aping an American accent. Not in the obvious way that Lonnie Donegan and Mick Jagger would sound anyway. Yes her singing voice is different from her speaking voice but I'd say it is more of a kind of non-accent rather than a specific American accent. Would some call it mid-Atlantic? I'd question whether she is putting anything on too. If I am singing a specific Scottish folk song then I obviously sound Scottish when singing but other than that I'd say I sound kind of neutral. For me to sing say a Beatles or an Eagles song in a Scottish accent I'd probably need to concentrate to specifically do that. It wouldn't be my natural singing voice for those songs. Whatever your accent is like in singing someone is going to object anyway. For their first TV appearance the Proclaimers were announced by Paula Yates as being "really weird" when the only thing different about them was they sung exactly the same as they spoke in every song. I do try and have some fun sometimes in changing songs in to something like my Borders dialect - as far as the rhyming will allow anyway. So it is much nearer to how I speak (apart from the odd word or two) but it is a much less natural singing voice - for me anyway.   https://www.youtube.com/watch?v=gAZvus5Hh1o