I was reading this article on South Korea and plastic surgery, and a friend mentioned about her professor's experience when teaching in South Korea. She said, when her students were told that "physical beauty is not as much important as inner beauty", the students stared blankly at the professor, seemingly unfamiliar with the concept of inner beauty.
This reminds me of a scholarly article that I once read, that mentioned briefly that not all societies are familiar with the concept of synesthesia - as we are with the concept of "inner beauty", i.e. using physical qualities (beauty) to describe non-physical qualities (kind, thoughtful, smart, etc). I haven't been able to find that article again though.
So where did the concept of inner beauty originate? Was it an European experience? When and how it did become more common?
I've been thinking about this question overnight, and I can't give you a definitive answer because it's not something I've researched, but there is a very clear line in the New Testament letter of 1 Peter:
ὧν ἔστω οὐχ ὁ ἔξωθεν ἐμπλοκῆς τριχῶν καὶ περιθέσεως χρυσίων ἢ ἐνδύσεως ἱματίων κόσμος, 4 ἀλλʼ ὁ κρυπτὸς τῆς καρδίας ἄνθρωπος ἐν τῷ ἀφθάρτῳ τοῦ πραέως καὶ ἡσυχίου πνεύματος, ὅ ἐστιν ἐνώπιον τοῦ θεοῦ πολυτελές.
(Quoted from Holmes SBL edition)
Translated: Let not [womens'] external adornment be braided hair and the wearing of jewelry and the wearing of [fine] clothes, but [let it be] that of the hidden person of the heart, in the imperishability of a gentle and quiet spirit, which is of great value before God
Anyway, here you have a Biblical text that makes a direct comparison between 'outward adornment' and 'interior [beauty]. The question then is whether this text is taken up and used as through the middle ages to develop this context. I'm not really well versed on Medieval reception of 1 Peter.
The reason I raise it though is the fairly sustained influence of Christianity on Western thought, which might form one basis for the answer to your question.