No, there isn't any job description as far as being a merit source--it's just a bunch of members who've been selected by Theymos (based on his judgement) to distribute merits.  There aren't even any rules, though sources have been removed for abusing the merit system.  I don't know what KPI is.

Oh, and since I think I stumbled upon some sort of account farming operation and cases of merit abuse related to that operation, I'm going to be very careful about which Newbie/Jr. Members I merit.  Ratimov gave a lot of those members in that thread a good deal of merits, and while it's commendable that he's very generous, I think it's better to be careful when handing them out to new members. 

Cases like the one I think I discovered (I'm still appealing to the community for help) by necessity involve newer accounts, and I've stumbled across them once or twice.  So if you wonder why I'm reluctant to just throw merits at every Newbie that makes a coherent post, that's part of the reason.