In the context of the sonnet, the central entity on which it revolves is the lifeless sculpture of Ozymandias. Seldom would have a poet personified a statue in a manner which highly depicts the life and times of the Emperor. The emotions are carved well into the statue, which is why the poet has used the word 'stamped' instead of 'sculpted.'
A stampede means a situation of chaos amongst people running here and there and as there are no reference to postage stamps and mystery, 'sculpted' is the right answer.