No, I am not actually that great a fan, although I do recognise that he is a great writer.
This isn't just about Ian McEwan, but he is a good example: You would think that people stop and think before they dubbed "crap" or "poor writer" an author who has been so widely recognised as not only good but great, but strangely, that is not the case for everyone.
"Anything creative has to be judged subjectively"
Not entirely. Think of architecture. You might like a building or hate it (subjective) and you may also assess whether it is built well or really poorly (objective).
"whether something is good or bad is down to personal taste"
No, that would be whether you like something or not.
Good/bad is an assessment of quality, and as such, should be objective.
"E.g. 50 Shades of Grey is one of the worst books I've ever had the misfortune to read, yet it's sold millions & millions"
I didn't say bestsellers must be great books and their writers geniuses
I pointed out the awards, worldwide recognition, nominations, fellowships etc that Ian McEwan had in his writing career, by people who thankfully know to make such quality assessments on objective grounds.