I'm not certain. I thought it merely meant 'someone who does not fit in'. Whether it's a harsh word to call oneself, I think would depend on the perspective of the person. As I said recently on this forum, I once called myself a 'nerd' in front of a girl, and she said 'You shouldn't call yourself that', implying that she felt it could only be an insult, but I then explained to her I had no problem calling myself that.
If you feel comfortable with the term 'misfit' for now -or rather, if you find it an adequate descriptor, as I know you would very much like to fit in with your peers- then use it. Go by what you feel when you hear the word, not by what anyone else thinks, even your mother.
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clarity of thought before rashness of action