I never said mediocre. Overrated/underachieving? For sure.
Chris Simms never excelled at UT. A very simple way to demonstrate that is he was never a first or second team all conference selection, by coaches or media. The two years worth of playing (if you discount 2000 when he was benched on two separate occasions), Eric Crouch, Kliff Kingsbury, and one Seneca Wallace were selected over him. You can scoff at that criteria if you want, but at the time those QBs were seen as better and more valuable players than Chris. Other relevant UT QBs that received such honors, Quinn 2nd team twice, Sam 2nd team once, Major one 1st one 2nd, and of course Colt and VY.
As has been pointed out, top 10 in passing categories for the career is not the gotcha you think. If you want to get into how his stats stacked up with his peers we can do that. For all the talk about the horizontal passing game of GD he never completed over 60% of his passes in a season. Never in the top 20 in the country in a season. Yards per attempt 53rd in 2001, he did rise to 12th in 2002. That was his best standing in any one metric.
He never won a conference title, famously made it to one conference championship game which was a disaster. His OU record speaks for itself.
Overall his tenure was disappointing, despite some great memories like Aggie victories and the Cotton Bowl over LSU. Overrated/underacheived.