Mentors don't have to be the most fantastic artist in the company. They can be artists who are consistent producers, can jump in and do whatever is required of them, and are respected by programmers and artists alike. Mentorship in its most idealistic sense is about teaching inexperienced people how to be successful in their field, and how to become a professional that everyone likes to work with regardless of their position or expertise. Some of the difficulty in establishing mentorship programs is that management may expect the mentor to spend a lot of time officially mentoring an assigned artist. Perhaps it is simpler to suggest that a junior artist observe the communication style and production methods of the mentor, and assign the junior artist to the mentor's team to make that relationship easier to establish.
Most companies have an annual review they conduct for employees.
It would be useful if reviews were more tailored to fit the job description, rather than generic, difficult review forms that do not evaluate properly. Part of the review form should include an area for mentoring. Objectives should include seeking out or providing mentorship. When an artist is not being challenged, doing too many mundane tasks but being a real trooper about it, it wouldn't hurt to delegate a little responsibility to that artist. Perhaps you are the lead artist on a game and you've saved the juicy tasks for yourself.