Question

Why does the nightingale refer the frog as a fairytale

Answer 1

The fact that the frog had taken an interest in her singing and offered to hone her talents had seemed too good to be true and like a fairytale to the nightingale

Answer 2

It is so because she believes in what the frog says as she does not trust herself (because she doesn't  have self-confidence) and refers to him as Mozart (fairy tale)  because she is so excited about a famous music critic teaching her how to sing.