The qualifications necessary to be a mentor, vary depending on the industry.
If you are a medical surgeon, then your mentor will be highly qualified. But in some industries, the mentor is not necessarily highly qualified. For example, in the Police, long standing police constables, often act as mentors to probationary officers.
The three absolute qualifications for mentors are: an extensive theoretical knowledge, combined with years of practical experience, and an ability to effectively impart knowledge and experience to the next generation.
Not all knowledgeable people are good mentors. It is the combination of characteristics that make successful mentors, such a rare breed.