Like the argument from antiquity, the argument from popularity is a form of the argument from authority logical fallacy. Namely, that because some belief is held by many people, it must be true. Stated that way it’s clearly shaky ground for forming an argument, but as humans we are shaped and molded by powerful social forces, and often look for cues from our fellows to guide and inform our actions. Therefore this is an easy fallacy to commit, where the beginning and end of the argument is that “Well, everybody else is doing it or believes in it.”
In a way this is like a brute force fallacy, where authority is claimed through sheer mass of numbers. But it is always and irrevocably the evidence that an argument must be molded by. That acupuncture, chiropractic, homeopathy, etc are believed by many to be effective does not matter when the clinical evidence comes up negative. If those modalities actually worked, wouldn’t doctors integrate them into their practices? The way out of that one is to play the conspiracy theory card, which is a whole other post.
A good argument has premises weighted in fact (derived via the scientific method) and constructed using legitimate logic. Only then can we speak of the conclusions as being valid, not when certain conclusions are proposed using the blunt instruments of popularity or common opinion.