In theory a mentor system is a really good thing. Sure you can find a lot of the info online with a quick search, but many people don't learn by just reading a guide or watching a video. Actually getting hands on experience with someone showing you the ropes can be of massive importance, especially in a complicated fight where they can point out exactly what you're doing wrong. Having actually present, human helpers also helps a lot for being able to explain things or answer direct followup questions. It might be easy to find a guide that tells you Do X To Beat Y, but what if you want to know why that works or what if you've tried that and it didn't work for you? What if you want to know what the next step is after Beating Y? A google search might not help much for that but an actual live person can.

The problem is, as others have pointed out, specifically applying special rewards to being a mentor. Having a crown icon, a mount and what is, for all intents and purposes, a special private chat channel, encourages people to just sign up and go through the motions. The requirements for being a mentor, and staying a mentor, are too lenient. If someone really wants to be a mentor they should have to do more than farm commedations, and if they want to stay a mentor they should undergo some form of actual review. It's a lot harder to do, yes, but it's necessary if they really want to have a designated mentor role.