"Mutability" begins with a broad pronouncement: people's lives, the speaker proclaims, are as changeable and fleeting as clouds crossing the face of the moon.
Take a look at the imagery the speaker uses to deepen this simile:
We are as clouds that veil the midnight moon;
How restlessly they speed, and gleam, and quiver,
Streaking the darkness radiantly! [...]
This vivid description presents the swift course of human lives as a celestial vision. These clouds aren't just speedy and restless, but also beautiful, lit up by the eerie "gleam" of moonlight and making the darkness "radiant[]."
Notice, too, the sweet /ee/ assonance that threads through these lines, making these "streak[s]" of "speed[ing]," "gleam[ing]" cloud sound as harmoniously beautiful as they look.
But all this beauty is fleeting and bittersweet. Take a look at the caesura that breaks into this vision in line 3:
Streaking the darkness radiantly!—|| yet soon
Night closes round, and they are lost for ever:
That emphatic break in the middle of the line stresses just how abruptly these clouds—and, by extension, people—can be swallowed up in the eternal dark "night" of death. Life, this poem's speaker imagines, is just a brief, shining vision, "lost for ever" almost as soon as it appears.
These first lines suggest that this poem is interested in the beauty of passing things—human life included. The fact that these clouds "speed," moving quickly over the moon and into the darkness, only highlights how lovely their gleaming, quivering, radiant forms are. If the clouds weren't fragile and passing, delicate as "veil[s]," they wouldn't be so lovely.
There's plenty to feel ambivalent about in that thought, however. The rest of this poem will examine both the power and the danger of change.