Hmm - Smith's "nature is her own mistress" seems a little aggressive to me given these other translations. I generally think Smith is worthy of deference but maybe this is one to investigate:

Bailey has: [1090] "And if you learn this surely, and cling to it, nature is seen, free at once, and quit of her proud rulers, doing all things of her own accord alone, without control of gods. For by the holy hearts of the gods, which in their tranquil peace pass placid years, and a life of calm, who can avail to rule the whole sum of the boundless, who to hold in his guiding hand the mighty reins of the deep, who to turn round all firmaments at once, and warm all fruitful lands with heavenly fires, or to be at all times present in all places, so as to make darkness with clouds, and shake the calm tracts of heaven with thunder, and then shoot thunderbolts, and often make havoc of his own temples, or moving away into deserts rage furiously there, plying the bolt, which often passes by the guilty and does to death the innocent and undeserving?

Munro has: [1090] "If you well apprehend and keep in mind these things, nature free at once and rid of her haughty lords is seen to do all things spontaneously of herself without the meddling of the gods. For I appeal to the holy breasts of the gods who in tranquil peace pass a calm time and an unruffled existence, who can rule the sum, who can hold in his hand with controlling force the strong reins, of the immeasurable deep? Who can at once make all the different heavens to roll and warm with ethereal fires all the fruitful earths, or be present in all places at all times, to bring darkness with clouds and shake with noise the heaven’s serene expanse, to hurl lightnings and often throw down his own temples, and withdrawing into the deserts there to spend his rage in practicing his bolt which often passes the guilty by and strikes dead the innocent and unoffending?