Timing is not relevant. A "bump" is when someone brings a threat back to the top without adding anything of substance, simply to move it up.
If someone has something relevant to add to a thread, no matter how old or young, they should post in the thread.
Zombies to me are old threads that died a natural death. They can be brought back by adding something interesting or relevant.
I think when some people get cranky about it is when the zombie is brought back for no good reason, and the thread title makes it seem like a current event or topic. In that case, it's basically a troll.