The Lestranges were thought ‘evil’, so they thought Leta was too.
It was very likely that people, including the students at Hogwarts, were prejudiced against Leta before they’d ever even met her because of her family name, Lestrange. When Queenie reads Newt’s mind, and heard the girl in his photo is Leta Lestrange, the first thing she says is that the family is “you know”, which likely refers to their notoriety in the wizarding world.
“QUEENIE
(reading his mind)
Leta Lestrange? I’ve heard of that family. Aren’t they kinda—you know?”
- Fantastic Beasts and Where to Find Them (The Original Screenplay)
This is confirmed by the executive producer of Fantastic Beasts: The Crimes of Grindelwald, who says that Leta is burdened by her family name, and her being a Lestrange makes people assume she’s a bad person because of the family’s reputation.
But according to executive producer David Heyman, Leta might simply be misunderstood. “Leta is in some ways burdened by her name,” he says. “The fact that she is a Lestrange has led some people to believe her to be a bad person. But as with a lot of J.K. Rowling’s writing, you have a rich complex character, and it’s one that Zoe brings to life.”
- Zoë Kravitz reacts to her character getting dissed in Fantastic Beasts (Entertainment Weekly)
Zoe Kravitz also mentions that the reason Leta is ‘edgy’ is because she’s used to being judged.
And from what I hear, she’s got a bad reputation?
Yeah, being a pureblood is a big deal. When she walks into a party people take a double take. She’s always felt like an outcast because of that and she’s very edgy because she’s used to being judged, and has created a bubble of protection of herself.
- Zoë Kravitz reacts to her character getting dissed in Fantastic Beasts (Entertainment Weekly)
One of the girls mocking Leta says how the name Lestrange makes her feel sick, implying that it’s a problem with the family that’s extended to Leta because she’s part of it.
GRYFFINDOR GIRL 2
I don’t blame them, she’s so annoying. Even the name Lestrange makes me feel sick—
LETA flings herself into their path, pointing her wand.
13-YEAR-OLD LETA
Oscausi!
- Fantastic Beasts and Where to Find Them (The Original Screenplay)
Grindelwald also mentioned her being despised among wizards.
GRINDELWALD
Leta Lestrange . . . despised entirely amongst wizards . . . unloved, mistreated . . . yet brave. So very brave.
- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)
So, it seems highly likely that people’s problems with Leta stemmed mainly from her family name.