Question

why did the nightingale called the frog ' Mozart 'in disguise?

Answer 1

Mozart was a renowned musician. The nightingale believed the frog to be epitome of musical genius. Hence, she called the frog a Mozart in disguise.

Answer 2

Innocent nightingale believed that frog was a great singer of the bingle bog . She thought the the frog would be a great singer like Mozart.
Thus nightingale believed the frog to be a Mozart in disguise.
Hope this helps...