Molly Cule's name is a pun on the word "molecule" hence the topic of this episode, and she was one of the original Frizzlettes that was led by the class's teacher and Bus driver, Ms. Frizzle.
As Ms. Frizzle's class washes Molly Cule's car before the concert, it is revealed that Molly Cule is a perfectionist when she notices the car appearing clean other than a tiny speck of tar that she uses a mega-magnifier she was given by Ms. Frizzle to prove it's still there when it's impossible to see without such a device.