in some frum communities denim/jeans are considered a “counter-culture” look, or a symbol of modernity. that is not the same as a tzniyus issue. attire that covers what needs to be covered & isn’t loud or too tight etc. should not be lumped into the category of “un-tzniyusdik”.
a knitted yarmulke is the norm is many communities. in others it can get you thrown out of yeshiva. does that mean there is something inherently wrong with the knitted yarmulke? I can’t imagine that anyone would say that.
you want to make rules against denim/jeans, go ahead. but be honest about the reason behind the rule and don’t proclaim that it’s part of hilchos tznius. it isn’t.