“Yeshivish” is a label. In todays cookie cutter generation we all want to be the perfect cookie, therefore, do what is expected to fit the mold and be branded wit this label.
The correct answer to the one who wears jeans when his parents wont find out is not “fear your parents” it is to SPEAK to your parents, respectfully of course. If you are uncomfortable doing so, speak to someone you ARE comfortable speaking with and have that person discuss speak with your parents.
The absolutely worst thing you can do is let it fester and when it comes to shidduchim, all will assume you fit the mold and you will date girls looking for the cookie. If you cant speak to your parents will you be able to open up to a girl who you dont know well and open up to her and explain who you really are? Will you get engaged to and then marry a girl who thinks you belong to one mold when in fact you are a square peg that was jammed into a round hole?
Your parents (I hope) want what is best for you, not their image in the community.