what evidence is there that jake does not approve of cohn boxing?

I don't know whether Jake doesn't approve with the boxing more than he sympathizes with it. Jake is somewhat sympathetic to the abuse and exploitation heaped on Cohn, and we intuit that Jake, too, must harbor similar feelings of inferiority. Though we know little about Jake's relationship with him so far, we will see that Jake is similar in some ways -- Cohn's flattened nose, for instance, foreshadows a less visible impairment Jake has (for Cohn, however, Jake maintains that the flattened nose has improved his appearance).

The narrator states that he is not impressed with Cohn’s boxing title and does not fully believe Cohn was the middleweight champion while at Princeton.