I'm one of the ones who is not a fan of Peggy.
It's mostly the way she is portrayed as *so* perfect. She's the kind of girl about whom adults would say "Why can't you be more like Peggy..." when you come in untidy, or grumble about chores, or trip over something, or argue with your siblings, or don't want to get out of bed in the morning.
She's daintily built and extremely pretty (without being flashy), always perfectly groomed, with shiny brushed hair, tidy clothes and ladylike hat, and the exact right amount of makeup use for CSland. She's obedient and sweet, with a gentle dignity, and not only always does things perfectly, but also takes it on herself to make sure everyone else is doing what they are supposed to. She works conscientiously, and gets good grades, without being bookish or overly smart, does well at sports without being a tomboy. She's domestic, and has no particular ambitions beyond helping her mother.
And there's an air she has, where she is completely sure the way she does things and views things is 100% correct, and the way everyone else should approach things.
It might also be that the first book I encountered older Peggy in was her eponymous book, when she first shows up as a deliberate contrast to how rude and disobedient and slovenly and generally unsatisfactory the Winterton girls are.
I much prefer someone like Bride, who is not particularly pretty and wears glasses and is a bookworm, and used to being a bit in her older sister's shadow. Or someone like Gisela Marani, whose is ladylike and well behaved and domestic in a much more natural way. Or even Len, who can be irritatingly perfect, but has an underlying thread of anxiety about doing things write, rather than Peggy's complacency.
In Peggy, the other prefects *do* warn Peggy about Eilunedd, but Peggy shuts them down pretty hard, and says
I don't understand you, Joan. It isn't in the least like you. You've always been so fair
and then laughs at them.