Through 4 games leading up to the Ohio State game, Martinez was completing over 61% of his passes, was averaging 263 passing yards per game, and had thrown 7 TDs and 2 INTs. He had also run for 234 yards and 3 TDs. He was coming off an 18 carry, 118 yard rushing performance against Illinois, a game in which he also threw for 327 yards and 3 TDs.
That's pretty good.
His performance against Ohio State was abysmal... as was the case for the entire team. He came back to complete 65% of his passes in a win against Northwestern the following week, but got hurt late in the game. From there, his production dipped. He only threw 3 TDs and had 4 INTs the final 4 games of the season (though he rushed for 4 more TDs).
Keep in mind that JD Spielman was also battling some injuries the final part of the season, we lost Maurice Washington, and Wan'Dale Robinson was banged up. Not having those players, and not having anyone capable of replacing Stanley Morgan's production, certainly contributed to Martinez's struggles. He also threw nearly 100 fewer passes in 2019 than he did in 2018, so his opportunities were much more limited (31.5 attempts/game in 2018, 25.1 attempts/game in 2019).
There's no doubt that Martinez could have played better, but he wasn't a disaster considering his injuries and lack of consistent playmakers around him. I think we all just expected (hoped?) a huge leap forward, and instead we got a step backwards.