Someone I barely know tried to cure my acne and it made me mad.
What about them trying to cure your acne made you upset?
I would rather not think about that because I don't want my acne to be cured.
It's understandable that hearing something about your acne from a stranger made you feel uncomfortable and upset. However, you should think about what their intentions might have been instead. Seeing that they meant you no harm will help you to feel better.
I'm pissed off because I don't want to look at the mirror ever again. They don't know shit.
I think it's reasonable that you're upset as the stranger had no right to give you acne advice when you didn't ask for it. Many people are insecure about acne and for them to assume you would be fine and open to their suggestions was quite rude. I'm sorry you had to deal with this intrusion.