People are upset because they feel betrayed. There was a lot of Holly worship, and she was made out to be the best dancer who ever danced in addition to thinking she was sweet and kind because she'd comment back on Instagram, etc. When it was revealed that she's just a narcissistic drama queen, the worshipers flipped.
Kelsey's backstory is Judy doesn't like her bc she barely has any technique, and Kelli doesn't like her bc she thinks she's stupid. Both know she's slow to learn choreo, but she's also one of the prettiest they've ever had, fans love her, so they're better of keeping her than losing her. They resent her.
She's not dramatic, not spoiled, or any more narcissistic than the millions of 20 somethings who live on social media. Don't hate her because she's beautiful.