Salome
you are what you is
I always reckoned that mainstream was more considered 'the common denominator'mainstream implies middlebrow -- that it's intended for the broadest audience possible and intended to maximize it's likeableness while minimizing it's offensiveness. it's by definition bland, and it appeals to the lowest common denominator. that doesn't make it low-brow, it just makes it safe and also conservative. it by definition is a sound
popular is just that -- popular. lots of people listen to it. it wasn't necessarily intended for the mainstream, nor was it necessarily intended for mass consumption, but something about it caught hold with the masses and many people chose to consume the product.
while popular is whether something is considered 'desirable'
according to this something can be popular or unpopular with the mainstream the same way as something can be popular or unpopular with the 'underground'
I would presume your definition is the correct one which would mean that U2 indeed has always achieved to be popular instead of being mainstream
they always have wanted to connect with as many people as possible
I don't think they consider any of their albums as a more artistic approach to achieve this than any of their other albums