I don't tend to think of things as 'overrated' as it's so subjective, but books I haven't enjoyed that most others seem to have include:
The Power - loved the concept but thought it was quite poorly executed and I didn't find it believable (eg a religious cult starting in 'this' day and age)
The Secret History - everyone and their dog loves this, especially avid readers, I just... didn't. Don't remember why now. The Little Friend was more interesting although too long, and I agree with others that The Goldfinch needed a good edit!
The Girls by Emma Cline - I found it totally devoid of atmosphere and just didn't enjoy the writing
The Girl on the Train - didn't expect to love it but have read sooooo many superior thrillers and still don't understand what about this one captured everyone's imagination!
I probably started with the wrong David Sedaris but people have always recommended him to me so I read Santaland Diaries last year and was actually quite shocked at how vile some.of the humour was (eg disablist). Probably just a product of its time though and I'll give him the benefit of the doubt.
How Do You Like Me Now by Holly Bourne - I thought there was a lot to like about it (eg its dealing with emotional abuse) but was so frustrated by the way a central character acted so out of character just to fit the story.
The Silent Patient by Alex Michaelides - I hated this. Just awful and felt really poorly researched (eg the way staff at a psychiatric hospital talked about their patients) and mysoginistic.
Anything classic that I haven't enjoyed (Anna Karenina and Life: A User's Manual come to mind) I've clocked up to me being too stupid 