Synne, the phrase “plain Jane” is familiar to me, but Meriam-Webster agrees with you.
plain adjective
\ ˈplān \
Definition of plain (Entry 1 of 5)
1 : lacking ornament : UNDECORATED
2 : free of extraneous matter : PURE
3 : free of impediments to view : UNOBSTRUCTED
4a(1) : evident to the mind or senses : OBVIOUS
it's perfectly plain that they will resist
(2) : CLEAR
let me make my meaning plain
b : marked by outspoken candor : free from duplicity or subtlety : BLUNT
plain talk
5a : belonging to the masses : COMMON
b : lacking special distinction or affectation : ORDINARY
6 : characterized by simplicity : not complicated
plain home-cooked meals
7 : lacking beauty or ugliness
8 archaic : EVEN, LEVEL
I disagree strongly with equating “plain” with the opposite of good-looking. Some of my favorite styles of made objects are very plain, emphasizing good materials and workmanship: Romanesque churches, Shaker furniture, types of Art Deco that highlight a curve or other simple shape. All of these are beautiful (to me) because they remove the distraction. Mid-century modern furniture, which does the same, is currently very popular. A plain outfit highlights the user’s own beauty without artifice, and can be very beautiful. Of course, if you’re going to do things—outfits, hairstyles, cooking, buildings, lamps, whatever—simply and don’t permit yourself to obfuscate, you need to have a good foundation.
Zaoebi, you look absolutely beautiful. Your accessories highlight your features for an overall elegant look.