I'm not sure that it's a "whim." Whether any of us like it or not, it's well-known and well-documented that all of us make all kinds of immediate, unconscious assumptions about others based on appearance. Is your appearance conveying the messages about yourself that you want to convey? The clothing I wear to work every day isn't necessarily what I "want" to wear -- it's clothing that sends the message about who I am, and my role at work, that I want to communicate to others at work.
I'm not saying you should change the way you dress, and I'm not saying that it's okay for your instructor to allow your appearance to affect your grades (if that is what is happening). But you might want to start thinking now about what kind of messages you're sending to others who don't know you well, and whether those are messages you really want
to be sending. Nursing school
is preparing you to enter an occupation where you will be interacting all day with strangers and professional colleagues, all of whom will be, to some
degree, whether it's right or wrong, making judgments about you based on your appearance.
Just something to think about ... Best wishes for your journey!