Cleary was a tremendously good writer who has a lot to teach us about characters and point of view and writing in general. (I wrote a book about her for Twayne Publishers way back in the 1990s and got to read everything; I had no idea she'd written at least one tie-in novel for Leave It to Beaver!)
Though when she first started writing she had details in the books that "dated" them (Henry Huggins wants a raccoonskin cap with a detachable tail, which was popular when she was writing the book; and one of her teenaged characters is reeeally focused on the little white gloves she'll wear on a date), later on Cleary managed to include details from the time period when the book was written, but still make them seem very fresh and new. Beezus, for example, goes to get her hair cut "like the skater on television"--which would have been Dorothy Hamill, given when the book was published, but which is still generic enough to be any skater the reader is aware of.
It was astonishing to read the Ramona books, because as a child I'd been so irritated with her while reading the Henry Huggins books, since I was seeing her from Henry's point of view. Seen from her own point of view, Ramona is an amazingly believable child character just trying to get through life; she's one of my favorite characters.
Cleary was just a wonderful writer whose books look deceptively simple.