If you got an item, it is as described, and it was shipped within the timeframe on the listing, negative feedback is not appropriate. Do not leave it.
However, neutral feedback might be in order.
Some more details might help here. How far in advance of your need did you make the purchase? How long did it take for the barrettes to be mailed, and was it within the time frame promised on the listing? Were the barrettes sold as a pair, or singly, and you bought two? Did the listing say anything about which side of the head it might be for? (I know of at least one superhero barrette seller who does specify it's for one certain side, unless you request otherwise.) How many times did you attempt to contact her, and in what time frame? (Four in a day, two in a week, etc.) Did you attempt to use the computer as well as the phone? Because the Etsy app is buggy. :/
If she went outside her mailing time frame, then that's not good. But if she was within the promised time frame, then she has not earned anything less than positive feedback. It's always great when a seller *can* speed up a shipment for you, but it's not always possible, nor is it required; that's why the time frame is given on the listings.
If the barrettes were sold in a pair, you have every right to contact her with your concern. She may have done it on accident, so be neutral. "I noticed they were both for the left side of the head. Since they were a gift, I didn't have to contact you about it, but I thought you'd want to know. Do you have any for the other side of the head? Can we work something out?" If she can't or won't help you out, this would earn her a neutral feedback, or if she ignores you, maybe a negative. (I'd still go with neutral if the barrettes were otherwise as described.)
If they're sold singly and you purchased multiples, sadly, that's one of those things where sides might not occur to you until you get them, but it's not the seller's obligation to make sure you think about it, either. Chalk it up to experience. However, you'd be fine in contacting the seller with something like, "Hey, it didn't even occur to me they might both be for the same side--in future listings, would you offer left/right as a variation? It might help someone else from making the same mistake I made!" (And with the new Variations, it's incredibly simple to do, at no extra cost.) And maybe ask if she can cut you a deal on a pair made for the other side. Then your recipient can wear them properly and have a backup pair!
And if they're sold singly and it states "For X side of the head, request for Y side," then the seller has no burden at all here.
In short: If they were sold as a pair and you got two for the same side, contact the seller and explain what happened; it's probably an oversight. See if you work something out. If she won't, or doesn't respond to you, it's neutral, maybe negative, feedback time. Explain it in the feedback: "Ordered a pair, but they were for the same side. Tried to contact seller to rectify, but seller did not respond after 3 days. However, they were well made, shipped on time, and quite lovely."
If they were sold singly, it's a life lesson for you, but letting the seller know about your confusion--in a polite way--might help *them* help future customers. And as long as they shipped within the time frame specified on the listing, and the items were as described, she has earned a positive feedback. You can always note the poor communication in the feedback text.