Hmm, maybe it does increase the critical hit rate after all.
Here's what I've gathered so far:
1. The Japanese in-game description of Zoom Lens says it increases accuracy under certain conditions.
2. The North American in-game description says it increases crit rate instead.
3. TRE hit 30 out of 30 times with Megahorn using a Zoom Lens and moving after the target. The target was being healed by an item, meaning that it was going first but not attacking the Megahorn user. Whether the other Pokemon in the battle (it was a Double Battle, I believe) were attacking the Zoom Lens holder is unknown.
All this leads me to my current hypothesis that Zoom Lens increases accuracy by at least 20% as long as the holder moves after the target. Again, I haven't done testing of my own yet (although it will probably be my next project). This is just what I've gleaned from various sources.