When it comes to esthetics, "plain" usually means bland or unexciting. A "plain sofa" means a sofa that no one will get really excited about, neither positively nor negatively. Ditto a dress. "Plain vanilla" doesn't mean it's bad ice cream either, just one without extra features.
However, when it comes to people, even more so with women, "plain" is often used in a way that implies ugly as in "plain Jane". It seems, to me, as if it is not meant to describe the person as average looking, but rather as ugly. A synonym, in this context would be "homely".
Is my perception correct? How did "plain" end up with an extra negative connotation, but really only when describing people? Was it originally a euphemism for ugly but has now just taken on that meaning directly?
As regards examples, here's what a sample of what a google search
"plain jane" ugly returns:
Louise Roe Gives an Ugly Duckling New Feathers on CW ...www.nytimes.com › 2010/07/28 › arts › television Jul 28, 2010 - “Plain Jane” borrows from the annals of generic self-help as freely as it borrows from “What Not to Wear,” though it rejects that series's snarky ...
Pretty Ugly Love (Plain Jane Love Stories Book 1) eBook ...www.amazon.ca › Pretty-Ugly-Love-Plain-Stories-ebook Jane is an unattractive girl with low self esteem and secretly hides the pure envy and resentment she feels towards her two stunningly beautiful best friends.
What It's Like Being The Unattractive Girl | Thought Catalogthoughtcatalog.com › stella-hui › 2014/09 › what-its-like-being-the-u... Sep 25, 2014 - I've come to terms with being a Plain Jane, but for most of my life I've felt flat-out ugly. My mother always told me that there are no ugly women, ...
Basically, I highly doubt any woman aware of the term would take plain Jane as a compliment. Or even as a neutral statement.
Now, I might be wrong on my perception of English North American usage, which is what this question is about, but this is the type of connotation that motivated my question.