I get that this is genuine and meant well, but telling someone not to worry about something is not always terribly helpful. OP's concern is valid and justified and telling someone they shouldn't worry about it doesn't mean they can stop worrying about it -- and if they can't, that can make them feel worse because now here's someone else telling them their concern isn't something to worry about. Guaranteed, someone has already told OP not to worry about this.

Also, yeah, people are definitely paying attention to her acne. I people-watch all the time. I would 100% notice it. That said, I would 100% forget about it once I'm looking at something or someone else. Saying "no one" gives that much attention to it reminds me of the "no one cares about fat people at the gym" thing. It sounds nice but it's not true.

I work with such beautiful girls who wear flawless makeup and they're so nice to me but I feel really ugly next to them

Maybe I'm unhelpful because all I have to offer is "I empathize; been there" but... well, yeah: I empathize; I've been there -- not with acne, and my insecurities stem from what I've done to myself, but I know the feeling.