How does 土口 match 'a person wearing a pendant'?
There is no pendant in the character. 「袁」 (Mandarin: yuán) originally meant to wear (clothes), where

「土」 is a corruption of 「又」, a picture of a hand, signifying the action of wearing clothes, and
「口」 is a corruption of 「〇」 (yuán), a phonetic hint.

「〇」 is the shape of a circle, and this word is now written as 「圓」.




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「袁」 was originally 「⿰衣又」, which is an abstract depiction of a hand 「又」 putting on robes 「衣」. This word is now written as 「擐」 (Baxter-Sagart OC: /*[ɢ]ʷˤren-s/).

「擐」 (to wear, put on) is made by

Adding on an additional semantic 「扌・手」

Complexifying 「袁」 to a pure phonetic 「瞏」 (something like /*[ɢ]ʷˁ&lt;r&gt;en/) by adding on 「目・罒」 to the top.


「瞏」 is systematically written as 「睘」 today across all characters.

「⿰衣又」 had a few variants, produced by some small modifications in two main ways:

Sometimes, multiple 「又」 were included, and 「又」 at the top was further corrupted into 「止」.
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Other times, 「〇」 (/*ɢʷ&lt;r&gt;en/) was added as a phonetic hint.
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「〇」 is the shape of a circle, and this word is now written as 「圓」.

Both of these modifications were eventually incorporated:
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Later,

「止」 merged with the top part of 「衣」 and was further corrupted into 「土」
「〇」 was systematically corrupted into 「口」 across all Chinese characters

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References:

季旭昇《說文新證》
殷墟甲骨文中の「遠」「（邇）」と関連字
小學堂
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》
許進雄《懷特氏等收藏甲骨文集》
中國社會科學院考古研究所《小屯南地甲骨》


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