In a comment, you mention:
The fact that it is blue and has white daisies is more important for my story. So I don't want it to attract more attention than: "Oh you're obviously from far away we don't have clothing that nice" (Thank you <PERSON> for the quote). But I also don't want the heroin to be (coincidentally) in a medieval play just before the time travel (explaining an outfit that doesn't attract attention).
Since you don't want to attract a lot of attention, you'll want the dress to be very conservative by modern standards. High neckline, long skirts, definitely not open in back and slit practically to the hips.
Just being "blue" is possibly going to attract some attention; although woad dye might not have been unknown, other dies, if known at all, would be known to be very expensive. There's a reason it's called "royal blue". (Worse, some colors might even be illegal for non-royalty. If your heroine runs into a merchant first, however, she probably has a decent chance that the merchant won't be inclined to hand her over to the local lord, and may give her more appropriate clothing.)
I totally understand why you "don't want the heroin to be (coincidentally) in a medieval play"; you'd need to do some hand-waving to make that plausible.
However, if you're willing to tip the scales in your heroine's favor in a way that's less implausible, you might have her wear a cyanotype-printed dress. 412
Cyanotype can be used on fabric (and cotton, specifically) to produce white patterns on a rich azure fabric, which can satisfy your requirement that the dress is blue with white daisies, and unlike modern, full-color prints which would be quite amazing to medieval people, cyanotype is much more approachable to your target time period. (The necessary alchemy may not be known, but the physical process is well within reach of the peasants of the day and not unlike wax printing, which your merchant may know about.) Moreover, if your heroine happens to have made the dress herself, she might even know enough to reproduce the process, which would probably be valuable to the merchant. (She'd have to have some pretty solid knowledge of chemistry for this, though.)
Being foreign is probably sufficient for your audience to accept the style of the dress. If your heroine knows about cyanotype, she can probably explain away the printing as being "special dyes from my homeland". Her garb, just from the quality and the blue dye, is clearly expensive, but perhaps not (literal) "king's ransom" expensive. Another plus is that cyanotype may not be as stable as more traditional modern dyes, which would make such a garment less "magical".
As for reaction, the merchant will probably assume she is someone important and/or wealthy, and her general state of health (and lack of calluses) will tend to support this, as will any evidence of education, especially if she happens to be able to read any of the period scripts (but this is unlikely unless she knows Latin). On the other hand, her knowledge of basic mathematics, and Arabic numbers (which started spreading through Europe right around the time you specify), may be valuable to a merchant.
Of course, as others have noted, you're going to have a whole other problem when someone sees what she's wearing under the dress. 412