This is from The Way of Analysis by Strichartz, chapter $7$, section $7.4$, page $276$. He writes
"In discussing power series it is good to recall a nursery rhyme:"
"There was a little girl
Who had a little curl
Right in the middle of her forehead
When she was good
She was very, very good
But when she was bad
She was horrid."
I don't understand why or how this nursery rhyme is related to power series.
Kudos to the comment by Barry Cipra. The rest of the poem can be seen here. I wonder if the rest of the poem can be turned into having some mathematical significance.
The nursery rhyme may be referring to the fact that power series are "very, very good" within their radius of convergence (absolute convergence, term-by-term differentiation and integration, uniform convergence, etc.) while they diverge outside the radius of convergence (which is "horrid" as far as series are concerned).
The curl in the middle of her forehead may be referencing the boundary of convergence; on one side, it's very nice while on the other side, it's horrible. (It's also a possibility that it's simply part of a preexisting rhyme, as @Blue puts it.)