Sites like Quora and Stack Exchange have robust reputation systems. Reviews on ADPList now are almost completely positive. People do not want to antagonize a mentor. Anonymity is not available like Glassdoor to provide anonymous feedback.
The goal is to have mentors/mentees differentiated to better help find good matches.
This is a complicated topic, but a robust reputation system would need to find a good balance to work.
My suggestion would be to have several facets of a review for both the mentor and mentee which they grade the other on a 1-5 system.
For mentors you have facets like friendly, knowledgeable, technique, strategy, process, testing, networking, big company/startup, and other qualities. Each with a 1-5 grade.
For mentees, you have creativity, energy, smarts, etc with the same grading.
Also a text area for details.
Lastly an option to make it anonymous.
After every grading, it would not register the review for a random number of days between 14 and 30. That way it would be harder to know who said what protecting anonymity.
Obviously, the design of this can go in many directions. This is just a strawman to illustrate the idea.