Similes add to the poem's eerie visual imagery while also telling readers quite a bit about both "Man" and the Man-Moth. Take lines 3-4, which feature two similes:
The whole shadow of Man is only as big as his hat.
It lies at his feet like a circle for a doll to stand on,
To the speaker, situated somewhere "above," the Man's "whole shadow" appears constricted, appearing below his feet in a circle no bigger than "his hat" and looking like a little platform meant "for a doll."
These similes reinforce the omniscient speaker's perspective: the speaker is looking at this man from above, and at a distance. Note, too, that the "whole shadow of Man" refers not just to this one man's literal shadow, but, metaphorically, to the influence of humanity itself—to the "shadow" that humankind casts. The similes thus further suggest to the reader that even in a man-made setting—a city—Man's control is limited. He is not quite as free as he thinks he is. He's like a toy, a prop, with little agency. Unlike the Man-Moth, who keeps trying to reach the moon, Man is passive and resigned, stuck in one place.
In stanza 3, another simile helps connect the Man-Moth to the figure of the artist or dreamer. As the Man-Moth climbs up the side of the building, his shadow stretches out behind him "like a photographer's cloth." This is another bit of striking imagery: readers can picture that Man-Moth's shadow flowing behind him like a dark cape, the kind a superhero in a comic book might wear.
And think about how different this is from Man's shadow, which just makes a little circle underneath his feet. The Man-Moth's shadow is elongated and "dragging." And while Man gets reduced to a mere "doll," the Man-Moth gets compared to a photographer, an artist who tries to capture or reveal something new about their surroundings. The Man-Moth is trying to do something that has never been done before. He has a unique vision, the simile suggests.
Another simile in the same stanza links the Man-Moth with the world of art, as the speaker describes the Man-Moth's dream of pushing "his small head through that round clean opening [the moon] / and be forced through, as from a tube, in black scrolls on the light." While an ambiguous image, this might bring to mind black paint being squeezed from a tube onto white canvas (or even the "black scrolls" of handwriting from a pen onto a blank page).
Finally, there are two similes in the last stanza that both relate to the Man-Moth's "tear." First, the speaker compares this tear, "his only possession," to "the bee's sting." The simile relays just how vital this tear is to the Man-Moth. A bee uses its stinger when it feels threatened. And, when it comes to honey bees at least, stinging something will kill the bee: the bee will literally rip its body apart when trying to pull the stinger back out. The Man-Moth, like its fellow insect, suffers when deploying this "tear," which the speaker goes on to say is "cool as from underground springs." In other words, it's as refreshing as cool spring water.
The tear might symbolize any number of things, including the Man-Moth's hope or perhaps a piece of artwork. Perhaps it even represents the poem, with the stinger simile conveying the deep vulnerability and pain that may accompany the creation of meaningful art.