They know a lot about people, what they are truthfully like, how they actually act when they think no one is paying attention.
Since people have little interest in wallflowers, they usually know a lot about people. The good and the bad.
Wallflowers are not mean, or impolite. They are usually pleasant, and respectful- just shy and/or introverted.
Nontheless, people tend to use the word "wallflower" negatively. What a shame.
Teacher- "Pass that paper to Jane."
Student- "Who's Jane?"
Teacher- "The girl that's been sitting two seats behind you all year. She's almost in all your classes."
Student thinks to himself- "It's not my fault she's a wallflower."