Despite the primary gloss here, it is far from certain that the word (in Aristophanes, Frogs 57) is affirmatory in intent. Dover (below) 197 surveys its range of possible meanings, here and elsewhere, and sees it here as an expression of either revulsion or distress.
Lament in tragedy sometimes contains elements such as papai, (p)appapai, attatai (e.g. Sophocles, Philoctetes 746, 754, 790), as well as Homeric popoi. Aristophanes has a whole series of parodies of these sounds. See at epsilon 2807, esp. notes 1, 14-18, 23, 25, 26.