JKR once complained about fans using the nickname “Voldy”. She said something like it makes him sound like a big fluffy teddy bear and fans should realize that he is seriously evil and dangerous.
So I found it funny that she had Peeves use that term at the end:
We did it, we bashed them, wee Potter’s the one, And Voldy’s gone moldy, so now let’s have fun!”
to which Ron remarks:
Really gives a feeling for the scope and tragedy of the thing, doesn’t it?