Are Devil Fruit users weak to salt?
No, this is just the special weakness of that particular Devil Fruit.

I mean, Luffy eats everything, nearly everywhere. And in the most dishes there is salt, or how do you cook you spaghetti?

Another instance to prove my opinion is that Luffy had gone underwater several times and had nearly drowned. In the process, he also "ate" salt, but it had no effect, like losing his Devil Fruit power.