I don't know, definitely was a bad day for John Lennon and those who cared about him but I wouldn't say it was a bad day for music - he already had a very large body of work by that stage, it's not like he didn't leave behind a large legacy. Imagine if he died in 1962 crossing the street to go to the studio or something.
Don't get me wrong, it's still sad that Lennon died, but from a "music history" point of view, I'd sooner mourn the death of someone like Ian Curtis who was also extremely talented and creative, but died before he got the chance to do anything much.