I think it's a fine requirement, because at least they're being realistic in telling guys that "Not every one of you will make it as an MMA fighter long term. And having at least the basic educational skills will ensure that you'll be able to land some sort of job if this doesn't pan out".
You see this with boxers a lot. The vast majority of the time, you have boxers coming from poor, impoverished backgrounds with minimal education with no education to fall back on. Just like in MMA, only a handful of boxers truly "make it". There are a few who save their money up and have something to fall back on after their careers, and then there are the vast majority who become burnout precautionary tales. Guys who never made it beyond the journeyman stage, and didn't have any kind of education to fall back on, but ended up hanging on for too long and now they're punch drunk fools. That happens all too often in boxing.
At least the UFC is trying to be up front about the chances of a fighter actually "making it" by telling them to at least have a HS diploma/GED to fall back on. They should be commended for that.