Are my walls too thin to keep creepers from arming?

Creepers can certainly get close enough to arm through cobblestone wall, but that's not the problem; they can even get close enough through a two-layer thick wall of obsidian; their arming range is a full three blocks. And yet, even a door is usually thick enough to stop them.

Creeper arming is not just based on distance, but also visibility. Cobblestone Walls are a block and a half tall for collision purposes, but not for Creeper line-of-sight. If the walls are only one layer tall, creepers have a clear line of sight over it and can still arm and fire.

Note that even transparent blocks like glass, glass panes, etc. block creepers' arming. And they can't see through the holes that form in Cobblestone walls more than one layer high; the blocks all still count as completely solid. However, the blocks aren't completely solid when they run up to most other blocks. So, while a glass wall,

a cobblestone wall,

and doors

are all fully able to stop them from arming, placing either glass or a door next to the walls gives them a gap to see you through:

Additionally, you can stop them from arming by getting much higher ground on them. If the top of their head is at least a full block below your feet, they won't arm.