Meaning of 自己被收拾得乾淨漂亮 in context and significance of passive voice
知道那天自己被收拾得干净漂亮


The author left out the subjects because they are very obvious.  The sentence 
will be easy to be understood if we add them back as follows.

(她)知道那天(她)自己被收拾得干净漂亮。

她 she
知道 know
那天 that day
她 she
自己 herself (not another girl)
被收拾得 be put in order
干净 clean
漂亮 beautiful

(她)(知道)(那天)(她)(自己)(被收拾得干净漂亮)
(She) (knows that) (that day) (she) (herself) (was neatened clean and beautiful)

She knows that she herself was neatened clean and beautiful that day.
She knows that she was neatened clean and beautiful that day.

收拾 means 整理 - to put things in order; to order; to tidy; to neaten.
For example, to clean the face, to comb the hair, to sort the clothes, etc.
It does not necessarily mean to apply cosmetics or to put on another more beautiful clothes.