What a great question!
My husband and I have travelled all over Europe, and spent a lot of time in Paris. It kind of funny, but lot of times we’ve had tourists approach us and ask questions of us in French. It's has been flattering, as I guess we look approachable as well< LOL We've been asked if we were Norwegian, Belgian, German and even Canadian, but never American.
I do think it may be because we dress a bit nicer, though not fancy, and as others have mentioned, we do not advertise our "tourist status" with a map or that over-awed, over-whelmed, over-tired look of disorientation that so many understandably have in Paris.
I would suggest nice, clean, pressed fitted jeans with a belt and a nice shirt tucked in, with a jacket or sweater, and maybe a cute scarf, or nice casual pants/slacks with cute shoes that you can walk in. Americans tend to wear tennis shoes, trainers, or other athletic shoes for regular use, but in my experience most Europeans, especially women, generally wear normal shoes. (i.e. I think shoes are a real give away)
Stylish walking shoes that are flexible soled, cushioned, but still qualify as cute shoes or cute sandles, not trainers are super choices. The same goes for clothes--Lycra (unless your jogging along the Seine), tank/crop tops, T-shirts, Flip-flops and generalized "play clothes" are dead giveaways.
So much these days, the "casual Friday" American-look tends to translate to “scruffy" clothes for vacationing, or traveling on a plane and people tend to start looking like they are going to the gym and that they don’t really care. I can attest to the fact that you really do get a lot better service too, if you look well put together, whether on the plane, at a restaurant, walking around Paris or strolling around The Met or Central Park in NYC.
Anyway, in general I think Europeans dress better than we do. Many of them have fewer, but nicer clothes than we do. And traveling to fashion forward cities like Paris are a great time to wear all those nicer things we all have in our closets, but maybe seem a bit too nice to wear around town here to grab groceries in. Enjoy your time there. I am confident that your interest ahead of time in what to wear, says you will be wearing something that is just right. It will be such fun to being able to feel part of the City and "look the part.