I came across this phrase in the cartoon Dexter's Laboratory(S01E11 06:23).But i have found no explanation about this either in a dictionary or on Internet.Is it kind of a nonce which is coined by Dexter and can only be seen in this cartoon?
Here's the clip you're referring to.
In this context, I would imagine "oh, my sweet pajamas" is used to exclaim "Oh my goodness!" or "Oh wow!" and yes, I would call that a nonce.