What does this sentence in my chapter book mean?

I'm reading a book named "The School For Good and Evil" by Soman Chainani. One of the sentences in this story reads, '"You wouldn't know Good if it crawled up your dress!" Agatha yelled.' What does this sentence mean? Below is more dialogue from the story if you need it.

'Everyone turned to Sophie. "We were both there! We were in his tower!"
"And I bet you saw the Storian too?" Beatrix sneered.
"Matter of fact, we did!" Sophie retorted to laughter.
"And did it start your fairy tale too?"
"It did! It did start our fairy tale!"
"All hail the Queen of Fools!" Beatrix proclaimed to roars.
"Then you must be the Grand Empress."
Beatrix turned to Agatha, arms akimbo.
"Ugh, The 'Mistake,'" Beatrix groaned. "Good has never been so wrong."
"You wouldn't know Good if it crawled up your dress!" Agatha yelled.
Beatrix gasped so loudly that Tedros cracked a grin.
"Don't talk to Beatrix that way!" a voice said -
Agatha turned to find blond-haired Tristan -
" Beatrix?" Agatha exploded. "You sure you don't want Tedros? He'd love to marry himself!"'

So, can someone tell me the meaning of '"You wouldn't know Good if it crawled up your dress!"''? Thanks!