The thing is, Troi doesn't read minds as such. What she does is sense emotions. That's not scanning anyone's brains so to speak. I don't think she'd be a parft of Starfleet if she was in any way unethical. Think about it. She's an officer on the flagship of the federation commanded by the most moralistic and ethical captain in Jean Luc Picard. If she was unethical in anway she'd have been reprimanded on more than one occasion.
You're right, Deanna most of the time is a very ethical person, but her mom isn't. I still wonder why her mother can act like she does, without needing to fear any consequences. How she specifically acts around Jean Luc borders on stalking and how she treats Worf is (almost) bullying.