These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Similarly, J.P. points out several of Lilly's faults, including the ways she mistreats Mia. We are later supposed to attribute this to J.P.'s Manipulative Bastard nature trying to pit Lilly against Mia, but his points are valid...
The entire Lilly/Mia conflict in the final books is a Base Breaker. Was Mia too self absorbed to notice Lilly's pain, or was Lilly irrationally blaming Mia for something that wasn't her fault?
Family-Unfriendly Aesop: The movie seems to believe that Beauty Equals Goodness - no matter how strong or intelligent or charismatic Mia is, she must look like a supermodel at all times or she isn't worthy of having authority or being noticed. Note that the villains in the first film are hideous and the film makes sure you're aware of it.
In the books, Mia points this out and mocks the movie creators about it.
Book example: Michael insists that Mia has to have sex with him in order to be a true girlfriend. She eventually gives in. When she walks out on him before they can do the act, even her PARENTS say that she did the wrong thing. The only guy who stands up for her decision turns out to be an inadequate boyfriend for her. Eventually she reconciles with Michael because First Guy Wins. The moral of the story? If you love him, sleep with him, even if he's fooled around with girls where it 'didn't mean anything' and he's being manipulative about the issue of sex and YOUR RESPECTIVE AGES MAKE IT STATUTORY RAPE.
Fridge Horror: Mia is fifteen years old in the film. The newspapers published pictures of a semi-nude fifteen year old and another of the same girl getting forcibly kissed.
Genius Bonus: Paolo says "I love your eyebrows, I shall name them Frida and Kahlo. If Brooke Shields married Groucho Marx, the kid would have your eyebrows."
Harsher in Hindsight: The books, mainly due to the cultural references, can veer into this. Like in the second book, where it's mentioned that Beverly Bellerieve is sitting in front of the World Trade Center for Mia's interview.
Hilarious in Hindsight: The part with Prince William in the second movie becomes all the funnier once the Royal Wedding comes to mind.
In The Princess Diaries: Royal Engagement, when Chris Pine's character is shown the "easy way" to win at darts, he objects with "That's cheating!". Cue Kirk and his approach to the Kobayashi Maru scenario in Star Trek...
Magnificent Bitch: Grandmere borders on this sometimes. At one point she puts on a school play to help Mia raise money enough to save the student government, ensure her own purchase of an island in Dubai, and get her granddaughter closer to a boy she thinks is more suitable consort material. She succeeds on all counts.
Suetiful All Along: Mia, mainly due to Sympathetic P.O.V., doesn't consider herself attractive, and describes herself in unflattering terms despite the fact that she is tall and thin with blonde hair and Gray Eyes, and is even compared to looking like a model at certain times. We, the reader, are meant to notice that she is actually very pretty and is attractive enough to get attention from several boys. Even Grandmere thinks she's pretty and wishes Mia could see it. By the tenth book she seems to have accepted that she is attractive and has taken effort in her appearance and fashion (mainly due to her friendship with Lana).
Unfortunate Implications: Critic Kevin Murphy - yes, thatone - noted in his book A Year at the Movies that the makeover in the film seems to have removed every trace of Semitism from movie!Mia's face.
Also Tina, Ling Su, Shameeka and other non-white characters aren't in the movie.
The girl credited as Marissa is supposed to be Shameeka (she's shown running the camera for Lilly's show, which is stated multiple times in the novels to be Shameeka's job). And Tina is Justified in the novels by saying that her father threatened to sue the studio if they put his daughter in the movie.
Wangst: Mia veers into this at times. Possibly justified in that the story is written from entries in her personal journal, and it would make sense for her to be inwardly over-dramatic. (This trope is the source of several Funny Moments in the series.)