I'd be dubious about a dressing with that much olive in it, especially as there's nothing to balance them. There's no acidity in there. You'd have to balance it by adding extra lemon to the fish, and then it gets tricky, especially as the fish is also oily. I would generally prefer something lighter and sharper for mackerel. That sounds like a good dressing for a rocket, tomato and feta salad, but YMMV.
Sam
Good, a Nay! My instincts go against it personally but the forum will decide!