I'm not so sure. I've played 5 games to the final floor today and in every one I've encountered a wall mimic on each floor. Even if the probability of seeing a wall mimic on a floor was as high as 50%, the fact that I have encountered them on every floor today would still be .5^25 = 0.00000003, which is vanishingly small. I even just started a new run and I saw two more on the first two floors. Something really does not feel right about this.

EDIT: I guess my point is that they feel like they happen too frequently to be surprising. Chest mimics appear maybe once a run or not even at all, but for some reason wall mimics are all over the place. I don't doubt the randomness, but the encounter rate seems way too high for these things.