To respond to Ladislaus --
It really depends on how immodest we're talking. If you're nitpicking, then NO, just shut up and mind your own business.
But if it's a parishioner who's been going there for a while, she should SEE that everyone else is modest, and YET she is not changing her immodest attire. At some point, it becomes obvious she is determined to greedily harvest attention from males as a resource of sorts. At that point, I think some social shaming is called for. Absolutely social pressure is in order. If the woman doesn't fear God, then social consensus is the only thing some women understand. It used to be socially acceptable to shame things that were shameful, and everything worked well. When you let women do whatever they want, some women will go crazy with it. I suppose that's true with any human being.
But I agree with Ladislaus, that if the immodesty is serious enough, the priest NEEDS to be brought into the loop. It's his job. If/when he fails to do his job, THEN you move on to the social shaming, firm but polite confrontation, etc.