If you do a quick search on the quote, you'll find that John Lennon, in all likelihood, never said that. Others have suggested that if it was actually stated by Lennon, it was said in jest. That would be consistent with Lennon's snarky sense of humor. If this was stated seriously by Lennon, it was later contradicted by other statements he made and actions he took.
Paul McCartney is an accomplished drummer! Here is a link so you can read about it: http://drummagazine.com/paul-mccartney-the-beatles-other-drummer/
It is certain that John Lennon was being perfectly frank and stating a simple fact which he had seen demonstrated many times. Paul McCartney is indeed the very best of all the drummers who ever played with the Beatles.