I think it was either Ozzy Osbourne or Alice Cooper, but I can’t find a cite either way.
Are you thinking Lon Chaney after Midnight?
IMDB says:
Jonathan Morrill Jonathan Morrill … Lon Chaney Vampire (as Lon Chaney)
No, I mean the now-lost silent film classic starring Lon Chaney. I seem to recall some rocker doing the top hat, bulging eyes, and scary teeth thing circa ~1970? But as I said, I can’t corroborate.
Well, Ozzy making a crazy grin would’ve been really close to Chaney’s character, even without make-up, so my bet is on him.
Alice Cooper has always been fond of top hats. I’ve rarely seen Ozzy wearing a hat of any kind.
Could you be thinking of Screaming Lord Sutch? He used to wear a top hat and cape, and sing horror themed songs (I think his biggest hit was Jack the Ripper)
A quick Google image search turns up pictures of Ozzy, Alice, and Screaming Lord Sutch all wearing top hats at some point in their careers. I don’t know if any of them were specifically referencing London After Midnight, but all of them could have been.