Another writer recently said to me, "sometimes you sound very brutal/honest. but nevertheless that also makes you a good mentor/guide." I would quibble with "brutal." But life's too short for treacly and insincere, when you are looking for real help to improve and grow. And if you lie to people just to make them feel good about themselves for a moment, you haven't really helped them become better writers. If you can do both - help people become better writers AND understand that lack of specific, learnable skills TODAY does not mean they suck as human beings, then you can be a writing mentor.