That's your issue. There are plenty of examples of Diggs coming up and making tackles, although it definitely is not his strength as a corner. He missed on that play, that shouldn't define him, but it does for too many of our fans.
Here's the play again just so you can enjoy it. Kittle was juggling the ball to make the catch. Diggs went for the ball instead of the player because he's always going to go for the ball. That caused him to completely miss Kittle as he pulled in the catch and moved inside of where Diggs was going. It was a poor choice because Diggs had a better chance of dislodging the ball with a big hit, but the way the NFL calls penalties on big hits, I can't blame him for that. He tried to throw a shoulder into where he thought the ball that Kittle was juggling was going to be but whiffed.
That does not absolve Diggs from other times when he's missed a tackle, but I don't see a lack of effort there. I see a really good catch that negated Diggs' plan of action. Just cause he whiffed doesn't mean he wasn't trying. It just made it turn out uglier than the effort.