Does this reduce damage like stone skin at all?

If it doesn't, can I ask why?

Even if it only happened after rank 40 when rangers face heavily protected opposition it would be really useful to have a small in-built damage reducer. We can't all get those talismans and scrolls that other rangers hog

Bark skin is not DR. It can situationally be (via edges), and serves as (not great) armor for unarmored slots with respect to armor deflection.

YMMV, but I don't think rangers really need always-up general DR.