Can I tell a girl she is ruining our class atmosphere?
That's mean. What will happen if you tell her about her effect on the class?
Some people don't like her and so I think it will be helpful.
If someone is disruptive in class, it's okay to feel frustrated and talk to the student into being better behaved during class, especially if you aren't the only person who feels that way. However, you must talk with the student tactfully and with understanding as it can be rude and hurtful to single out someone aggressively.
I'm not trying to ruin her life. I'm going just let her know why people treat her that way.
Well say it nicely or better yet leave her alone. Singling her out will make her feel bad.