I was going to mention Roseanne's "Star Spangled Banner" but someone beat me to it. But then according to Johnny Carson, she was actually mouthing along to the sound of a goat being neutered.

Well, there was Pat Boone's cover of Judas Priest's "You Got Another Thing Comin'".

Who did that horrible cover of "Whiter Shade of Pale"?

There was that Lord's Prayer that Cliff Richard did to the tune of "Auld Lang Syne". That nearly made me grab a high-powered rifle and sit out on a freeway overpass.

Finally, anyone who picks on Shonen Knife, whose covers are the best, is no friend of mine and obviously has no sense of humor or style. You could probably listen to Portsmouth Sinfonia without cracking up. Now, if they did "How It Makes Me Quiver" or whatever it's called THEN you'd have a point. But then some songs are so bad they suck no matter who covers it.