Yes, there are servers online. Once there is no 6. version people play the 5. one, so, at the moment, there are 4 ZOMB servers.
Unfortunately (or fortunately) Teeworlds 5.2 have a low number of players so if someone play ZOMB with you, consider yourself a lucky guy.
And Answering for Pi_, no. Like goo wrote (and far as I know), there is no source available, so no one can port the gametype to 6.x versions.
The only way to have ZOMB Mod under 6.x is create a new Mod, and no one will do that... 'Till now?
EDIT:
I'v tested now zChaos. It's a beta test. Woof.
I would like to have it on my computer.
(Like Elefant said: It have waves and bots/zombies).
I hope it became public soon cuzz it really reminds Zombie Mod.
Playing Teeworlds since 2011!"I will always be topless for you" - Günther Branlutte