I would NEVER walk up to a woman I did not know to point out her non-tzniut dress. Talk about not tzniut! In fact, I wouldn’t even do that for someone I DID know. Come to think of it, I wouldn’t even do that for my wife!!!
Given that according to many opinions I’m not even supposed to tell a woman that she is wearing shatnez, an objective issur from the Torah, kal v’chomer for tznuit which is subjective.