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.
Designated Villain: Janine is demonized by Claudia for... it's hard to say, her motives change randomly. Probably because Janine has the unfortunate tendency to be obnoxious (with an IQ of 196, this makes sense)
Alan Gray for Kristy, Cokie Mason for the whole BSC on occasion.
Jessi's Aunt Cecilia qualifies. She is definitely right about how an eleven-year-old, regardless of how responsible she is, should not be left in charge of an eight-year-old and a toddler for a whole weekend. (She's wrong, however, in that she seems determined to blame Jessi for the matter, rather than Jessi's parents who left her in charge.)
Pamela Harding in the Little Sister spin-off series is meant to be a pint-sized Alpha Bitch, but ends up coming across as this. While she does have some genuinely snotty moments, the main reason Karen started demonizing her was because in her first appearence, at Karen's sleepover party, she didn't want to eat pizza, didn't like The Wizard of Oz, and preferred to sleep in a bed over sleeping on the floor.
Ensemble Darkhorse: Shannon is very popular among members of some snark communities. Amusingly, so is background character Pete Black. Janine also has a following.
Logan is also rather popular, he even got two "Reader's Request" books narrated by him.
Andrew in the Little Sister series.
Fad Dissonance: It's pretty hilarious to read about Claudia's totally weird new friend who wears long dresses and flared jeans, both of which have come back into style since the books were published.
Fan Preferred Couple: Claudia's sister Janine and Kristy's older brother Charlie are this for many fans.
"Funny Aneurysm" Moment: In the series finale, the girls make a time capsule to be opened in seven years and each contribute a letter. In Dawn's letter, she talks about flying between California and Connecticut and remarks that maybe by the time the capsule is opened, air travel will have changed somehow. The book was published in 2000.
Hilarious in Hindsight: In Mallory and the Mystery Diary, published in 1989, Mallory complains that it feels like she's been 11 for a decade. Cut to 1999, when the books are still being published and poor Mal is still 11...
Ho Yay/Les Yay: Lesbian subtext is a popular topic of discussion among fans. Some would also argue that Jeff Schafer has quite a bit of "chemistry" with Mallory's triplet brothers, particularly Byron.
Nightmare Fuel: Claudia sits for a family in which the husband abuses the kids. After Claudia and her mother rescue the children and their mother, Claudia gets a phone call late at night. It's the abusive father, wailing at her to give him back his children.
It's probably easier to catch the Loch Ness Monster than it is to find fans of Karen Brewer.
Snark Bait: Several blogs and online communities are devoted to snarking these books.
Tear Jerker: The death of Louie, Mimi's stroke, deteriorating health and eventual death, and a lot of Abby's autobiography.
Jessi's baby brother, Squirt, getting hurt in a car wreck.
Unfortunate Implications: Every description of Mallory and Jessi includes their similarities and differences. The problem? One of the girls' "differences" is that Mallory is white and Jessi is black. No matter who is narrating, they use this comparison EVERY SINGLE BOOK.
Jessi's from the only black family in town.
The book about the autistic girl was full of this.
Stacey, due to her sad backstory, being frequently hospitalized, and dealing with her parents' divorce.
Jerkass Woobie: Karen Brewer if you really consider what she's had to live with. Her parents divorced and remarried within a year and she was apparently expected to adjust pronto despite being six years old at the time. Then Watson and Elizabeth dropped a "surprise! We adopted a baby!" bomb on the whole family. Her behavior may be inexcusable at times, but it's hardly surprising.