yup, it's a kind of corn, not an expression. :)

It is a variety of sweet corn.

Although this is a resolved question, I must say that Jasmine is correct. While "Sweet niblets!" is not heard too often in every day speech, it is a colloquil expression. However, rather than merely meaning "My Goodness", it is used as a substitute for profanity. Especially the phrase "Oh Shit!" It is used in this way in practically every episode of the TV show "Hannah Montana" by Miley Cyrus.

i am sorry that some people think i am wrong but the expression of sweet niblets is meaning oh my goodness... it may also mean a kind of corn but it is also an expression here in america... and hey i think i know my own country and language people ok i have lived here my whole life and it is my first language. sweet niblets as an expression means oh my goodness.

sweet niblets is like saying "oh my goodness."