I don't think Oda has ever used niya niya for a D. smile (it does overlap the jolly and big smile implication).
I kinda link "niya niya" when the character is smiling slyly, the char has a secret side motive and that smile comes before the char's goal is achieved (or meant to be achieved) XD That's what I feel from niya niya, but didn't really find such a specific definition to be sure But this way it might explain, why it was never used when the D. char was smiling - their smile is often honest and very cheerfull instead of being mischievous only. If Nekomamushi was shown with niya niya, it might be a pattern typical for cat, you can never trust a cat
I've always find プリプリ (puripuri) funny. It is implying someone being angry, but it sounds so happy and fluffy, so I laugh at the contradiction. Most of the time the sound description matches.
Wahah, contrasts are always what makes me laugh most of the time I know puripuri from moments filled with freshly cooked food, makes me hungry everytime >.<
Is it intentionally comedic?!
I don't know the origin. I'm thinking "jiggly jiggly" is very similar if not the same "puri puri", so that is a potential overlap/mix up. Ah, this is from Pokemon (so might just might be me remembering the name of Jigglypuff from the Japanese games/cards which is Purin).
And now I was reminded of purin, Japanese puding....which actually has its own character XD