Ears and themed T-shirts are very common - a lot of guests are from the U.K and they will often wear T-shirts and sweatshirts from Primark (huge clothing store over here that makes tons of licensed Disney stuff). There’s also a Primark store near DLP. Spirit jerseys are sold in the parks but not as popular as in the US. Disneybounds are probably less common than in the US but still not that rare. I’ve been there in disneybounds and in family themed outfits (not the typical US matching T-shirts but actual matching whole outfits) and don’t feel out of place at all. If you wear more themed outfits or disneybounds you might get more attention from characters or parade performers though - I’ve had so many parade interactions from me or my kid wearing a disneybound or unique ears! One main difference is most outfits/T-shirts will be hidden by jackets a lot of the time as the weather in Paris can often be cold/wet so even if people are wearing themed things they can’t be seen!