Layer more armor? sure. But heavy armor is heavy, and you need to cover the whole ship since you can't know what part will be shot at. Turrets and gatling guns are both *very* cheap, and reasonably light so you can stack tons of them on there.
This is a point I'll disagree on. One of the major parts of ship armor design (And really any design involving armor) is engineering yourself with limitations for your task.
If you're going to be designing a Destroyer meant for attacking a Station or other Heavy Warship, you'll likely want your Bow heavily armored for taking the brunt of punishment from Slow Moving or Immobile targets. You can afford to be less armored on your Aft side because the targets your designing around aren't likely going to be able to capitalize on that.
As just an offhand real life example, you can look at Tank design. Or, perhaps far more relevant, ship design.
https://en.wikipedia.org/wiki/Category:Naval_armourAdding more firepower to counter heavy armor is a perfectly reasonable strategy, however (And this is extremely the case for small fighters) is every weapon is also a vulnerability.
Each Turret is a block with less defense than a typical light armor block, and (Unless you're a masochist or only designing ships to work in Creative mode) an armor-less conveyer system beneath it.
And while each Gatling gun is a very cheap block that can en masse chew through heavy armor, you do have this balanced around fact that the Turrets you're flying into will chew through undefended weapons much quicker, at a farther range.