The Veela bit has already been touched upon, but that’s only the tip of the iceberg.
Fleur’s also got a few ‘trying’ traits that likely offended Molly, Ginny, and Hermione:
She knows what she wants, and won’t accept any less
Example: her comment about pink bridesmaids dresses clashing horribly with Ginny’s hair.
I highly doubt she MEANT to be offensive — she was just speaking her mind. It hadn’t, as yet, occurred to her that a different culture might’ve approached the same situation differently (with more tact).
I’m not French, but in my culture, there’s no concept of ‘saving face’, as we’re usually very direct in person and can come off quite rude (a stark contrast to our fellow countrymen of differing ethnicities). I’ve got Parisian friends who are very similar, and we had some explosive rows before we learned to get on (they’re nowhere near as diplomatic as Fleur can be).
To an individual who finds directness a particularly alien concept? ➔ Recipe for disaster.
She’s unapologetic about her taste
It’s heavily implied that Fleur’s got great taste — too many people, this can come off as snobbery.
Example: her talking about the Christmas decor at Beauxbatons, and comparing it to Hogwarts’ far more homely ornamentation.
Was she wrong? No — she was making a very valid (though exceedingly unpopular) observation.
We all do.
For example, I think Heineken is p!ss beer.
pictured: p!ss beer from quite a while ago (it was an outdoor music event, and the closest thing to water I could buy with a token 🤷🏻♀️)
However, city-dwelling hipsters in my country of birth take to Heineken like flies to a lump of *you know what*. Waste of our foreign exchange, but hey — it’s ‘fancy, imported beer’, so it’s got to be quality, right?
Nah. Nee. Nope.
We’ve all got things we’re ‘snobby’ about, whether they’re opinions on beer, books, decor, pets…etc. It’s human nature, and to act as though you don’t make you more sanctimonious than saintly.
She’s got b@lls enough to be herself
She doesn’t bend over backward to ingratiate herself with people.
Example: it'd have been incredibly easy for her to’ve trotted around going ‘yes, Molly/yes, Ginny/yes, Hermione’.
They’d have quite liked her — right up until the moment they came to realize that’s NOT her real personality, and that they’d been taken in by an illusion.
Wide grins, wonderful fluttery sweetness, being polite to a fault — you know whom that initial impression reminds me of?
Needless to say, Fleur isn’t Umbridge — nor was she ever Umbridgean in nature.
Molly, Ginny, and Hermione disliked Fleur because they’d never met anyone like her before and had some adapting to do before they could all get on (to be fair, so did Fleur).
Personally, I’d love a friend like Fleur.
Or, I should say, more friends like Fleur.
PS — No, I’m not saying Fleur’s perfect. If that’s what you’ve come to say, well ➔ I will not pander to commenters’ mistaken assumptions/petty dislike for a character/differences in opinion. There’s space for that in your own answer.