About denim skirts: they CAN be, and often are, quite tzniusdik in cut. And a few years back Shabbos denim suit, in a dark, soft denim, were the rage. When I told my kids no they mentioned east coast relatives who dress much more tzniusdikly then we do.
I have no issues with men in jeans, davening in jeans etc., as long as they are “kempt” vs. unkempt, i.e. tucked in, shoes tied, etc. But I haven’t seen similar conservatively styled jeans- that is trousers, not skirts – for men, or women, for that matter. And as long as they’re not regulation black rayon/cotton/wool etc. blend there will be a large number of yeshivish people who will look down on them.