To pull something off is to be able to succeed in something. When it comes to clothing, to be able to "pull off style" means to achieve the look. Being able to pull something off also means being able to remove something. So, if you can't pull off tight jeans, it means you can't remove the jeans from your legs, or you can't achieve the look you were aiming for.
I try wearing tight jeans , but I can never pull it off .