Are these synonymous to you? Not to me. I find myself saying about a woman (to myself) “she’s plain but not ugly” or “she’s definitely ugly, but not homely” but I think people tend to use them more or less interchangibly sometimes.
Just now, I saw a music video of some French singer (can’t tell you who, it was playing in the gym in French and I didn’t catch many details) who was extremely homely, but not at all plain. That is, she had a big nose and awkward hair and was all sorts of flamboyant, but not totally unattractive, either. (I think that was the point of showing off her long legs throughout the music video.) She was homely, but not plain or ugly.
Madonna, to me, is ugly but not homely or plain. I find Anjelica Huston very homely but not really ugly. I can;t think of a “plain” woman whom you’d all know, but “plain” implies a woman who can’t wear clothes or do her hair but who, even if she did know how to do these things, still wouldn’t be attractive for some unknowable reason. I’ve had crushes on plain women, but I always knew that I’d find it hard maintaining my sexual interest beyond a certain point.
Is this just me, or are these universal principles?