I wouldn’t say that at all. Google or Bing for photos of Garbo, or Bessie Love, or Norma Shearer, Joan Crawford, any movie star.
The style from about 1916 to about 1935 was for flat chests—so maybe the women don’t look like “bombshells” to some modern eyes. But “plain”???
I sure don’t see the dazzling beauties w high cheek bones and classic beauty. Look at the lineup of chorus girls in the stills that flash by on YouTube during the song. I think things began to change in the 30s when Hollywood started to get big and have outsize influence.
Not to mention, Billie Dove or Dolores Costello.
But I do think, recalling from the comments of my grandfather and his friends and peers, they did actually cast a wider net in terms of what they felt constituted beauty and attractiveness, compared to nowadays. It incorporated a certain hard-to-describe element of feminine sturdiness and poise, as opposed to the kind of sloppy, bosomy sexpot look that increasingly took over in the post-war years.