As far as I know, the legalities are that you are not allowed to PUBLISH a parody of a copyright work unless you have the permission of the author or copyright-holder. I personally think this is a bit tight-arsed (unless somebody's making money out of it and I don't get any), and I have derived much amusement from parodies of some of my songs which I have come across over the years. The mighty Les Barker's version of "Punch and Judy Man" springs to mind -

"Mrs Capstick, Mrs Capstick, where has your son Tony gone?
"The Irish Sea is six-foot-three, and Tony's only four-foot-one. . "

I have to confess that I am partial to a piss-take myself (let him who is without sin cast the first writ. . ), so I won't be suing Herga Morris any time soon. . .