My argument is that somebody who does not participate in Endgame Activities is arguably, not suitable to be 'Mentoring' new players, and is most likely going to give out false information.
To become a mentor, and to help newer players grow into better players overall, you should at least have a clear understanding of the combat or crafting side of the game.
Some basic ideas below, to make becoming and keeping a mentor status more strict.
Trade Mentor:
1. Completion of all current Gathering Nodes and Crafting Recipes per patch release (Yes, that means Expert crafting and Timed nodes too!)
Battle Mentor:
1. Completion of all current Extreme Trials per patch basis. (Yes, you lose your crown until you complete the latest trial on extreme)
2. Completion of at least ONE current Raid Tier on Savage as of the X.1 Patch of an expansion. (Your previous crown will last until the release of the first raid in the expansion).
3. 100 PvP Victories in Crystaline Conflict
Full Mentor:
1. All of the Above.
2. No prior warnings/actions taken against their square enix account by support.
Having much stricter requirements will easily weed out the Mentors that only have the crown as a collectable item, and clean up things like Expert Roulette for newer players, having only mentors that actually know what they are doing, playing in those activities.
Obviously these are extreme values, and not actual requirements, but so far the discussion has been good and there has been a legitimate reply with a much better mentor system that has been currently implemented.