I feel like basically being in the 50th percentile of anything means you're pretty "mediocre". The second synonym that pulls up for mediocre is average, btw. They literally mean the same thing.
As for the bolded... I feel like Montez has received a lot of excuses and slack from people for "mediocre" play because of "where he is in his career". The guy just finished his 3rd year on campus and 3rd full season as part of this football program. That's 3 years of S&C, 3 years in the same offense, 2 years behind one of CU's all timers at QB, he's saw what it took to be great in 2016, he's had big performances and really, really awful performances, both on the road and at home, etc. etc. etc. He is not a True Freshman being thrust into the spotlight in the National Championship game or anything..... Cause, you know, that never happens, amiright? The excuses of Montez being young and inexperienced need to stop.