another word for an effortless look as it relates to fashion
I am taking C. Z. Guest as the personification of the quality the OP is describing. In a comment, the OP said he wanted a word for a fashion company. Instinctive would fit, and Instinctive by Skoon has a ring to it. Instinctive, defined by Macmilan Dictionary
done without thinking, because of a natural tendency or ability
In the OP's sentence:
Her look was so instinctive, like she didn't care to look perfect but somehow still did
See C. Z. Guest, Photos; Photo below by Slim Aarons; Book by Susanna Salk
Just look at C.Z. Guest. She is not wearing shoes, her shorts are wrinkled, her only adornment is a narrow pink belt, but she could not be more perfect. She may have put a great deal of thought into that outfit and to her more formal clothes, but the effect was of instinctive ease and elegance.
Granted, it would sound a bit redundant in your second example sentence, but I think carefree would work well overall, including in your first example:
She was admired by everyone for how she looked, carefree, like she didn't even try but was styled impeccably.
carefree
adjective
without anxiety or worry.
requiring little care : carefree fabrics.
(from Dictionary.com)
As for its use as a name for a fashion brand, be careful, though, for “Carefree Fashions {of Scottsdale, Arizona}” might already exist (from etsy.com) and it could possibly risk confusion with “wash and wear"/"wrinkle free” fabrics.
Perhaps the word you need is insouciance. Merriam-Webster says it means lighthearted unconcern and is synonymous with nonchalance.
She was admired by everyone for how she looked, insouciant, like she didn't even try but was styled impeccably.
Incidentally, back in the 1640s when I was but a wee lad, Robert Herrick captured the sentiment you're describing in his poem Delight in Disorder. The poem ends:
A careless shoe-string, in whose tie
I see a wild civility:
Do more bewitch me, than when art
Is too precise in every part.
Wild civility is a magnificent oxymoron, and you could do worse than to choose that as the name of your firm.
I think "natural" fits your description best. It's a humble word, but all beauty in nature is arrayed in that way.
She was admired by everyone for how she looked, natural, like she didn't even try but was styled impeccably.
Her look was so natural, like she didn't care to look perfect but somehow still did.
https://www.thefreedictionary.com/natural