During the Thirties many WW1-built Battleships had complete turret re-designs to increase the elevation of their main guns. Instead of receiving hits on your heavy armor belt from a (relatively) flat-firing WW1 naval rifle, now you are taking hits directly on your deck from a relatively high-angle. Dive-bombing aircraft present a comparable threat.
Hood's only virtue was showing the flag and looking intimidating. It was a good sea boat and a handsome ship, though not nearly so handsome as her predecessor Tiger which was perhaps the handsomest of all the first and second generation dreadnoughts.
Even with the refits during the 1930s, primarily to add torpedo bulges and improve anti-aircraft defense, it never approached the level of protection it would have needed had it been at Jutland!
“It’s also worth considering that WW1 dreadnoughts (and Hood was a WW1-design BattleCruiser), did not take into account the “plunging fire” that it would encounter from a WW2 Battleship like Bismark. Hood had very thin deck armor as originally constructed.”
That’s the whole point. Hood had very thin deck armor because there was very little plunging fire in WWI when she was designed. At the ranges that you could expect to score any hits, almost all fire would be coming onto the side. With the advent of radar direction, and improved rangefinders, and greater elevation on guns, suddenly you could engage at much greater ranges, and hence the shells are coming down at a different angle. At the same time, the improved accuracy of gunfire did away with the main advantage of Battlecruisers, the idea that speed equalled protection. Suddenly that was no longer true. Basically Hood was designed for a different war and never received the comprehensive refit that would have set her up for WW2.