So no matter what kind of monument could be erected to atheism, you're going to just deny what it is? That's fine I guess. I'm still not sure what the point is - if your contention is true that it is not possible to erect a monument to atheism, what does that imply? I don't see that it's really that meaningful, even if you were right.
I would like to see a monument that reflects and glorifies atheism, in the same way as crosses and chapels reflect and glorify Christianity, statues of Buddha glorify Buddhism, and flag-carrying American Eagles glorify the USA.
In a peevish moment I suggested that a coil of rusty barbed wire would be a good monument to what atheism brought us. But I am not an atheist, so my artistic choices don't count.
There was some imagery recruited by the atheists to the service. There is a novel that pretends to describe a victim to the Holy Inquisition, "The Gadfly". There is Giordano Bruno burning on the stake. These were exploited by the comissars to what they are worth. But I don't recall monuments to Bruno or the Gadfly in the Soviet Union, perhaps because Bruno was, of course, a God-fearing Christian and Gadfly, er, did not exist.
The reason it is important is philosophical: atheism if about what is presumed not to exist. Hence it has no symbolism and no motivating power. Since people drawn to atheism are still people, they fill the need to glorify their worldview just like the rest of us. Some, -- not all -- turn to ugly bumper stickers, which is the topic on hand.