Why is Rachel Zegler so hated?

Because she is out of touch with what the masses want from films. Essentially a mouthpiece for Disney who have made failure after failure of films. She also slammed the original film for no legitimate reason, which is one of the most beloved children's films for many people.

Everything she said was correct. The prince was a stalker and she literally just said she found the forest scene scary as a kid. She is way more talented than her peers who cannot even sing live on set whilst keeping emotion etc.

Hardly a stalker, and it's part of grimes fairy tale. Changing the source material will always get fans in a uproar. Personally I don't care, this film isn't my cup of tea but knowing what netflix did with the witcher which was one of my favourite games it's a similar thing.

A useless actress and a loose cannon when it comes to making clickbaity comments most likely to antagonise the movie's targeted audience.
If the movie doesn't generate much in the way of paying cinema audience numbers or positive reviews in 2025 within a few years her career may go the same way that Pauley Perrette and Shannen Doherty's acting careers did.