To Kill A Mockingbird was recently voted the best novel of all time,
which is a shame because it's not very good. It's incredibly simplistic, for one thing. Mrs. Dubose's story is like an afterschool special. And it's insufferably folksy and preachy. Atticus comes off as awkwardly as Lawrence Fishburne's character in Boyz in the Hood.
It's aged poorly because for a book about not being racist, it comes off pretty paternalistic these days. And there are 20th-century books that deal with race in ways that haven't aged poorly; go check out Native Son, Their Eyes Were Watching God
and Invisible Man
and then get back to me.
But it's also just not a great book. It's okay YA reading, but it doesn't have the depth or the skill to be a great classic.