They clearly aren't great at playing zone defense and yes, it's the weakness of this team but it's all relative.
First, using total yards allowed is a bad indicator because style of play (they've decided to play deep and keep everything in front of them essentially) and game conditions (opponents throw a lot because they can't run and they are behind a lot) can influence it.
If you want to isolate and measure the skill of defending the pass, look at yards per attempt and yards per completion. OSU is a respectable 30th in the former and 37th in the latter.
Certainly room to improve but I don't think it's a crippling weakness against a team like Purdue who can only hurt you with one guy. Bell will probably be the latest opposing WR to go for 150+ yards but then again, if they've practiced it at all, they should be able to contain the damage.