Perhaps the best way to sort out clueless mentors farming mentor roulette not to help but to get a mount.
Introduce a system where you have to maintain a commendation level over a certain period.
Let's say 70% commendation rate.
A player can then use a negative commendation in place of a standard commendation, knocking down the commendation rate percentage.
The mentor will then lose access to mentor roulette until they are able to bring their commendation level percentage up again.
I know some will say this system can easily be trolled, and perhaps it can. However, I can't imagine enough people would use the system in a negative way to truly impact the stats.
I suppose it could also be introduced outside of mentorship too, but I don't really see the point. Those commendations won't mean a thing and a negative commendation wouldn't mean a thing.
The system may have to be tinkered because I assume DPS tend to get fewer commendations overall. I tend to always commend the DPS, especially if they are new and listened to mechanics, but I know most others default to the tank or the healer. Perhaps mostly the healer as I average 2 commendations a dungeon as of late. Never played as a tank.