At the signatures, they recommend you spruce up your park attire to look more business casual or preppy, at least. So, no crazy print T-shirts or jean shorts or flip-flops or tanks. Don't look like you're coming from the beach at the likes of the California Grill or Artist Point, ect.
I wear dress shorts or khaki style shorts or cargo shorts and a polo style collared untucked shirt. I still wear sneakers.
So, you don't need to go crazy at the signatures dressing up, but just more presentable and neater. And it should be pointed out that even though there is a dress code guideline that Disney asks it's patrons to adhere to there, there are always guests who don't abide by it at the signatures and Disney still lets them in.
But I can only speak for myself that I feel better about myself at places like Jiko and Monsieur Paul if I am dressed a bit nicer and it is a sign of respect to the people who run the restaurant as well, especially the French signature restaurant where the waiters are in kind of suit like tuxedo outfits as I recall.