If everybody improves their looks, the net effect will be zero. It's also not clear what parts of "looks" actually have an effect. It's also not clear what effect GLP-1 has on other elements that might make someone more employable.
The study itself is certainly not conclusive. The control group were women who wanted to take GLP-1, but didn't. That can indicate all kinds of status/class differences. The study also seems to have been a fishing expedition, and is based on self-reports, so it's tentative at best. I wouldn't recommend taking GLP-1 for a job interview based on this article.
>If everybody improves their looks, the net effect will be zero.
I'm not sure that's true. I think there's a baseline issue here. Regardless, even if it netted out to zero, you'd end up with a different order of who gets hired when, so for any individual it'd still make sense to try to improve your looks.
Same reason you shouldn't try to build skills or acquire unique relevant experience as part of your job search: if everybody does it, the net effect will be zero.