That's not really true. He is above average in QB rating. Above average in passing yards. Above average in completion percentage. Above average in TDs. And way above average in terms of wins. There isn't a good argument that he isn't above average but I'm sure you will make one anyway.
There isn't a good argument?
This year, Prescott was 15th in total passing yards -- 21st in passing yards per game (ie, he was higher in total passing yards only because he played more games than some of the guys ahead of him in ypg).
He was 10th in completion percentage.
He was 16th in passing touchdowns.
He was 14th in QB rating.
He was 18th in QBR.
He was 17th in yards per attempt.
He was second to last in sacks taken.
And he only resuscitated and salvaged those statistical finishes after a a blazing finish, relatively speaking, after a start where he was near the bottom in all these stats.
So yeah, actually, there's a very good argument.
He's essentially a middle of the pack QB in every major statistical category.
If you want to throw in his intangible and his strong finish, there is maybe an argument to be made -- not that its even going to be relatively convincing to most non-Cowboys fans, but at least you could make a good faith argument -- that he's around the 10th-15th best QB in the league. You can call that above average, maybe.
But in my book, he's decidedly average. I can name at the very least, 11 QBs who are unarguably superior to him (Brees, Mahomes, Wilson, Ryan, Watson, Wentz, Goff, Luck, Brady, Rodgers, Roethlisberger). I can name another 3-4 who generally put up superior passing stats, but who kinda have loser labels, and you could argue you prefer Prescott because of intangible (Stafford, Rivers, Newton, Cousins), but many around the league would consider vastly superior to Prescott. Then I can name a couple of other guys who for various reasons all have arguments that could put them in front of Prescott as well (Mayfield as an ascending rookie, Trubisky as an ascending 2nd year starter, Flacco as a Super Bowl winner, Derek Carr for being in a dumpster fire, etc). I mean, it's reasonable to make a list with Prescott as low as like 18th on it. That's approaching BELOW average range.
I wouldn't go that far. But I can't put him in the above average range either. He's decidedly pretty average at everything.
If he ROUGHLY duplicates his final 8 regular season games over an entire season next year, he would join the Wentz-Goff-Watson category, which is the true above average group. Still behind the elites like Mahomes, Brees, Rodgers, Brady, Roethlisberger, IMO, but in that next tier of ascending stars. But he has to extend that finish over a whole season. If he does, he'll deserve his $100m contract too.