My much longer reply got deleted, so I'll try to be shorter.
Dolezal is being herself not because she was born a liar, but because she became one over time and rather than evaluate, and try to become someone else (i.e. stop lying to get things) she continued to do so. She learned that lying works, and was uncritical of who that made her as a person.
JBY is advice that encourages tunnel vision, and an undue faith in the rightness of who we are at a given time simply because it's who we are right now. That's what makes it bad advice.
Dolezal believes in the validity of her attempt to 'become black'. She believes that rather than being a liar, she's 'trans-racial'. What she sees as transitioning to being black, the rest of the world sees as blackface and costuming, and then lying about it. Hence, as long as she continues the charade (blackface, costuming, paying a black man to pretend to be her dad) she is a liar. That said the pretend father and the lies about her adopted siblings make me think malice might be involved too.