Many Rabbis wear a long Black Coat called a frock as rabbinic garb. Since it is a rabbinical garb is it permitted for someone who is not a rabbi to wear it? is it haughty to do so?
There are many communities in which the custom is to start wearing this either from bar mitzvah or from being married. That being the case in such a community it would have nothing to do with being a rabbi. If one were to attach to it a special significance of only being for Rabbis as a specific custom perhaps you could make that case but I see no advantage in doing so
The Sefer Shulchan Aruch Hamiddot says that since a frock traditionally (especially in Litvish - Lithuanian) circles is a special Rabbinic garb. If someone who is not qualified, due to lack of scholarship or lack of position of authority chooses to wear one is exhibiting the behavior of Gaava - haughtiness.