It depends. My husband and I had a discussion about this not too long ago. Our son is in braces right now.
When I was a young child, and when my parents were kids, braces were uncommon, and most people had imperfect bites. When I was hitting tween and a little older, it seemed to explode and EVERYONE was getting braces, so it seemed. It almost seemed like it was a "popular" thing... you were "out" if you didn't have them.
Now I am noticing that almost every kid seems to cycle through getting them.
So, is it a popular thing to do? Is it vanity? Is it necessary? Why now and not before? etc etc.
I think it is a little of it all. My personal opinion. People of my parents generation (and hundreds of years before) didn't have access to such dental care. And their teeth showed it. No, not everyone had ugly teeth or ugly smiles. But think about how many of the older generation have bridges or partials. Why is that? Maybe because their teeth were never properly aligned, which contributed to worn teeth and dental/jaw problems that precipitated the losing of teeth, requiring dentures.
Or maybe not. How can you prove such a thing? You really can't.
I have never had braces, but my dentist (when I first became a patient in his office, as an adult) asked if I had--because my teeth were so nicely aligned. My husband is my same age (44) and a few years ago lost a tooth (just crumbled) and had an implant put in. He has another tooth that something similar is expected (but we are hoping it doesn't happen). He has some crowding and other things with his bottom teeth, and also never had braces. His smile/teeth are not unattractive, but I'd bet his teeth could have benefitted from having some orthodontic work when he was younger. The tooth loss now, could very well be related to having his teeth wear due to poor alignment.
Our son has/had a terrible overbite (couldn't see his bottom teeth almost at all). His teeth have never been unattractive or looked bad, but his overbite and the way they aligned, would likely cause long term damage (AND, he was biting into the gum behind his top teeth with his bottom teeth)... so he is in braces to correct it. Hopefully, he won't have more serious problems down the line, since corrections are being made now.
Daughter, has several teeth that have come in sideways, and also has a slight overbite (not as bad as son's). She, too, will have braces soon.
Do SOME people get them for cosmetic reasons? Probably. Does it mean there is no benefit if that is WHY the person ultimately decides to receive treatment? Not necessarily. But cosmetics could be what convinces someone to go forward with treatment, when otherwise they might be unaware of any long term detriment to themselves without treatment.
The way I see it, it is something a person needs to do a cost/benefit analysis for. What are the likely costs of treatment (time/$/pain) and the benefits of same. And what are the costs of opting NOT to have treatment (dental problems down the road that may be very costly, and have few treatment options, some permanently unpleasant/less attractive smile in the short run if cosmetics are on the list)?
Only YOU can decide if the tradeoffs are worth it. Your daughter needs you to be an advocate for her, since she's a minor. The early teen years are the time span when orthodontics are the easiest, because of the growth they are going through it makes the shifting of teeth/jaw bone growth much easier and faster.
Good luck with your research.
ETA: For what it is worth, I think the bullying is a non-issue when it comes to braces. SO many kids have them it just isn't an issue. Unless she is already a target for bullies for some reason, the braces aren't going to be a reason to suddenly kick start someone bullying her.