It was free online for four years first
Jeff Kinney put the first entries on a children's education website called Funbrain in 2004, back when the internet was still mostly text, a page at a time, for nothing. Something like twenty million people read it there before any publisher was involved, which is a slightly awkward fact for an industry that spends a great deal on deciding what will sell.
He had been working on it privately since 1998. Six years of drawing before it went up, four more before the first printed book in 2007 — so the overnight success took roughly nine years, which is the usual arrangement. The early chapters are still findable online, and they are recognisably the same thing, drawn slightly worse.
He wrote it for adults
Kinney's original plan was a nostalgia book for grown-ups, looking back at how mortifying being twelve had been. His publisher read it and said it was a children's book, which he initially resisted and then accepted.
That origin explains a lot about the tone. GREG is not a hero, is frequently a bad friend to ROWLEY, takes credit he has not earned and rarely learns anything by the last page. Kinney has been open that Greg is meant to be flawed rather than likeable. Children find this hilarious and recognisable; the adults who object to it are usually the ones who wanted the book to be improving.
The format is doing something clever
It is designed to look like an object a child made. Lined paper, a hand-lettered font built from Kinney's own writing, DRAWINGS in the margins, short entries that stop when the joke does. A reluctant reader opening it does not see a wall of text — they see a NOTEBOOK, which is much less intimidating than a novel.
And the CARTOON panels are not illustrations of the prose; they often carry the punchline the words have set up, so you have to read both. That is a real reading skill dressed up as a DOODLE. Teachers and librarians spent years being sniffy about the series and then noticed which children were finishing books.
The Cheese Touch and the rest of the grid
The single best invention in the books is the CHEESE TOUCH — a slice of cheese left on the basketball court that nobody will remove, and a playground rule that anyone who touches it is contaminated until they pass it on. It is a perfectly observed piece of MIDDLE SCHOOL anthropology: an arbitrary rule, taken completely seriously, enforced by everyone, that no adult is aware of.
The rest of the grid is the FAMILY and the surrounding cast: RODRICK, the older BROTHER with a band, MANNY, the toddler who gets away with everything, FREGLEY next door, and ROWLEY, who is a far better person than his best friend and completely unbothered by it. Greg's own COMIC strips, his SKETCH-es and the WIMPY JOURNAL itself fill the rest. Kinney has said he draws deliberately badly — the figures are simple enough that a reader finishing the book usually wants to try one, which is not something most novels manage.