We ended up discussing her dating experience at length at the time. Honestly it was plain ridiculous. Sure, many of the OKC messages were short "hi" or other thoughtless one-liners. But the content didn't matter, she treated them all as signals for a live one which drew her to their profiles where she would then hold a little impromptu trial of eligibility based on their photos and profiles. Most of the time she wouldn't respond, and it had nothing to do with the content of the messages. It's the photos, age, race, married/divorced status and religion that matter to her. The message notification just made her look.
What I saw was the polar opposite effect to "confidence-destroying". She became increasingly superior and picky with all the interest. And when she did go on real dates, she would find the smallest flaw as fatal because she had the impression that there were an effectively unlimited supply of competitors.
It does sound as if she got "gamified", taking pleasure in the app giving her pings. Such things afflict most of us in one way or another (including me). I suspect she'd find it more satisfying to seek out thoughtful men who are interested in her as a person -- but like I said, she should do whatever she wants.
My understanding of women's take on dating apps is, of course, heavily informed by the fact that they were usually dating me, and selected for that.
Likes are cheap; they literally cost nothing. So are message that say "hi", with no indication that they've even read your profile. Logging into a dating app and discovering thousands of people -- all of whom seem to know only that you're female and therefore approve -- is as soul-sucking as getting no attention at all.
I was very popular on online dating sites, because I knew how to talk to women as if they represented something other than a place to put my dick. All of the women I spoke to were incredibly discouraged by quality of attention they received. Ask your sister what fraction of those stacked-up notifications consisted solely of the word "hi".
Most women seem to get tired of the attention that merely tells them that they're attractive. It's an ego boost for a while, and I'm sure some retain it, but it's the kind of thing you get inured to, and you want something else.
The experience for women is different, but not necessarily better, and the things that a man might crave simply because he doesn't receive it will feel empty to a woman precisely because she does. There's a cultural asymmetry that doesn't lead either side to what they're after. But men can do a lot better simply by treating women like human beings, and understanding that they've got problems different from yours.