It's definitely a good guess, but I asked the author when the manga started and he said it didn't mean anything and it was just a word that he came up with as a kid and sorta stuck with him (maybe based on hearing about the papilla thing in science class?). But then they definitely don't use "パペラ" to refer to it, but 乳頭 (nyuutou, and even that is not common Japanese).
Also unrelated (but related to my previous point) I should note that he didn't get paid to do this manga, it was all on his free time.