From what I heard in a shiur given by Rebbetzin Samet, not wearing denim has nothing to do with it representing any zionistic group, but the fact that denim is know to be a material worn by the working class doing menial work. As b’nei melochim, we try to rise above that.
I think that the biggest problem I have found with long denim skirts (and maybe corduroy as well) is that they are very NOISY when walking in them. But as far as covering all necessary parts, they do the job just fine.
I have read on some of the more halacha-oriented posts here on the CR that there are some poskim (perhaps the Chazon Ish) who did hold that ankle-length skirts are best, but I think the ultra-chareidi (is that like a chumra on a chumra?) circles usually want skirts that are 4 in below the bottom of the knee because this is not a length that is affected by fashion. I do know plenty of very frum women who do wear their skirts to their ankles, so I don’t know.