Should strike tiles suffer from diminishing returns in cascades?

That's exactly the point. Defense tiles offer so much less utility than strikes that they desperately need the different treatment. They'd still be near-useless against active powers, but at least they would be an effective counter against match damage from cascades.

As an aside: The Thing is a bad joke. I've just fought him in Sim yesterday evening. Totally forgot about him and attacked another teammate. He jumped in front after the first hit and took the rest of the cascade. By the time he was going to put out his defense tiles, he was dead.
Sure, it was a lucky cascade, but the Thing is the worst tank in the game.