How about the mentor needs to describe in their own words why they would make a good mentor?
You might see I have no reputation on Stack Overflow, no code in Github, but I've been writing web code for 16 years. I might have a few things to teach someone, but you wouldn't know it from my online reputation scores in the usual places.
Even if someone was a coding rock-star, they might not make a good mentor just by that skill alone. Some of the best coders are quiet and anti-social, and trying to get information from them can be difficult. I'm not saying all are, or even a lot, but I've known some very smart people who struggle to answer "why did you do it this way?" in a way others can understand.