The number rating is a composite of everyone who ever read the book that decided to leave a review. If you look at the written reviews for Ishiguro you’ll see that a lot of the 3* ratings are people who consider his writing good but didn’t find it resonating with them personally and a lot of the 1* reviews are from people who wanted something different/didn’t resonate with how the story was told and as such found it boring.
I do use goodreads, but only to look at the reviews themselves. The number is meaningless as people take anything less than 5* as being crap rather than a 3* meaning that it’s probably pretty good but maybe a bit niche. The type of people who review are twofold - fans of the book wanting to enthuse about it, or people who feel like they didn’t get the experience they wanted and want to say that. Sometimes those people are reasonable in reviews, sometimes they just slap a 1 star on it and move on.
I personally don’t bother with books I’m not enjoying, but I find scheduling times to read and keeping to it helps if I’m struggling with a book I need to read. I have this thing where I cannot start a task if I don’t feel I have enough time to do it, so I put off things like reading when I get busy even though I know I can read for just five or ten mins if I wanted. Seeing the blocked out time for it helps a lot and means I can keep going with my reading goals even when it’s a bit of a struggle.