I feel like I'm repeating myself man. Just read my comment from 3 months ago, I believe I explained it better there. But I guess I'll try again (again) (again).
It's BOTH. She's a main girl who isn't initially portrayed as a prostitute. The rumors only appear FROM HER AND ONLY HER, so when she eventually says she's a prostitute (and it's then made 100% irrefutable shortly after when she exits a love hotel.) people are disappointed, frustrated, angry, devastated, etc. The buildup would've softened the blow, substantially, hence why I say it's necessary.
IF the rumors were propagated throughout the game as conversations you either have or overhear, then people would begin to deduce something, they'd begin to piece things together and make their own deductions about it. Once again repeating myself, THE RUMORS ARE ONLY TALKED ABOUT BY CHIKA HERSELF. So when she confirms the rumors ALMOST IMMEDIATELY AFTER INTRODUCING THEM and then SHE IS SEEN EXITING A LOVE HOTEL (making her confirmation 100% irrefutable) that's going to upset people.
Here's an example that's mildly exaggerated (replace 20+ hours with 5 to 10+ hours to remove the exaggeration): You pursue a specific love interest's route in a VN throughout the entire game. She's, in your opinion, the hottest character in the game. Then after almost 20+ hours of gameplay, picking all the right options, and dealing with all her trauma/story, you finally get the option to ask her out, so you do. She then proceeds to tell you that she isn't interested because she's 100% a lesbian and has absolutely no interest in men. Surely this is false, right? About 10 minutes of gameplay later, she then introduces you to her girlfriend and actively makes out with her in front of you. You are COMPLETELY DISAPPOINTED and FEEL LIKE YOUR TIME HAS BEEN WASTED. You spent HOURS of your life getting invested in this characters story, only to then have your hopes shattered when she randomly reveals something that completely changes her character, frustrated you replay the ENTIRE GAME and find ZERO CLUES TO HER IDENTITY.
(If you find "lesbian" to not be a valid example and as such it hinders your understanding, then replace the word "lesbian" with ANY OTHER CHARACTER TRAIT THAT WOULD RUIN SAID CHARACTER FOR YOU. For example: replace "lesbian" with "has a penis" or something else)
Does that example help you understand why people are mad? Or should I just give up completely? Also, once again, I recommend reading my comment from 3 months ago as it is sufficiently better written as I was not moderately upset from repeating myself nor was I sick (got the flu, but it's basically gone now).