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.
In Meg's defense, Executive Meddling forced her to break up Michael and Mia, and she was a mess while writing it, more so after the editor who mandated it left the publishing company.
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.
If it helps, Tina also points them out more gently, namely about Lilly getting Green-Eyed Monster about Mia getting attention from guys. Also, while Mia gets Lilly back as a friend, they aren't as close as they used to be, and Lana has been a much better friend to Mia.
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?
An even bigger example is the scene where the maids attempt to distract the Grandmother for the Princess... by singing "Frère Jacques", dancing, putting pots onto their heads, and then taking them off and clanging them together while singing, "DING DANG DONG! (clang!) DING DANG DONG! (clang!)" Its absolutely surreal in its own right, comes out of nowhere, its never discussed again, and seems so random and out of place that the sheer "WTF face" shown by Julie Andrews is mimicked by everyone in the audience who sees that scene.
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.
No. That is NOT the Aesop at all! The Aesop is that Mia wasn't emotionally ready for sex, since she didn't behave in the most mature fashion (screeching and tossing her snowflake necklace at him). Mia only initiated the sex (that never happened) because she thought it would keep Michael in New York, which was what Grandmere told her to do, and Grandmere isn't the most ethical source of advice. Michael later admits in Book Ten that he screwed up their relationship by not being honest about Judith Gershner, and his mistakes came to haunt him when Mia started dating JP during the two years he was away. He had actually expected her to stay single, instead of seeking a rebound guy like she did. They both needed a bit of growing up, and only reconcile because Mia is still attracted to him and it's not fair to JP to date him without that spark. More problematic is that Michael has no qualms about winning Mia back while she is in a relationship with another guy.
Paolo: I love your eyebrows, I shall name them Frida and Kahlo. If Brooke Shields married Groucho Marx, the kid would have your eyebrows.
In the second film, Lilly calls Brigitte and Brigitta Rosencrantz and Guildenstern.
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...
Stan Lee's cameo in the second film, which actually predates Disney buying Marvel.
Jerkass Woobie: Lilly becomes this in Book 10, after revealing that J.P. slept with her and dumped her specifically to date Mia after Mia and Michael broke up. Mia even says that while J.P.'s terrible behavior doesn't justify Lilly's actions, it makes them understandable.
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).
Unnecessary Makeover: Dovetailing with the Family-Unfriendly Aesop, the Hollywood Homely Mia has to get a makeover because she has glasses and a ponytail curly hair. On the other hand, she's clearly unhappy with her appearance until her makeover. And bear in mind, her makeover was more suited for a public figure than a high school student.
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.)