As far as wearing hat and jacket on hot day in Bronx Zoo, the Mishna Berurah says that one who lives in an area which is excessively hot can even daven in short pants (Begadim ktzarim byoser). So if one can even go to shul in shorts, al achas kamah vkamah one can go to the Bx Zoo in long pants and a shirt, without hat and jacket. I am not recommending one wear shorts to davening or any other time, I am just saying what the halacha requires.
To each his own. However, as I mentioned before, if non-frum Jews are led to believe that black hat and jacket are a necessary part of yiddishkeit, they will be far less motivated to ever become frum. Kol hamosif goreya.